Light-emitting element, light-emitting device, electronic device, and lighting device

ABSTRACT

A novel light-emitting element is provided. Alternatively, a novel light-emitting element which can achieve both high efficiency and a long lifetime is provided. The light-emitting element includes a light-emitting layer between a pair of electrodes. The light-emitting element includes a first light-emitting layer and a second light-emitting layer. The first light-emitting layer includes a fluorescent material. The second light-emitting layer includes a phosphorescent material. A difference in peak value between a first emission spectrum of light from the first light-emitting layer and a second emission spectrum of light from the second light-emitting layer is 30 nm or less.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a light-emittingelement in which a light-emitting layer capable of emitting light byapplication of an electric field is provided between a pair ofelectrodes. One embodiment of the present invention relates to alight-emitting device, a display device, an electronic device, and alighting device each including the above light-emitting element.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. Alternatively, thetechnical field of one embodiment of the present invention relates to aprocess, a machine, manufacture, or a composition of matter.Specifically, examples of the technical field of one embodiment of thepresent invention disclosed in this specification include asemiconductor device, a display device, a liquid crystal display device,a light-emitting device, a lighting device, a power storage device, amemory device, a method for driving any of them, and a method formanufacturing any of them.

2. Description of the Related Art

A light-emitting element having a structure in which an organic compoundthat is a light-emitting substance is provided between a pair ofelectrodes (also referred to as an organic electroluminescence (EL)element) has attracted attention as a next-generation flat panel displayin terms of characteristics such as thinness, lightweight, high-speedresponse, and low voltage driving. In addition, a display using such anorganic EL element is excellent in contrast and image quality, and has awide viewing angle.

In the case of such an organic EL element, electrons from a cathode andholes from an anode are injected into an EL layer, so that currentflows. By recombination of the injected electrons and holes, the organiccompound having a light-emitting property is excited and provides lightemission.

The excited state of an organic compound can be a singlet excited state(S*) or a triplet excited state (T*), and light emission from thesinglet excited state is referred to as fluorescence, and light emissionfrom the triplet excited state is referred to as phosphorescence. Thestatistical generation ratio of the excited states in the light-emittingelement is considered to be S*:T*=1:3.

In a material that emits light from the singlet excited state(hereinafter referred to as fluorescent material), at room temperature,generally light emission from the triplet excited state(phosphorescence) is not observed while only light emission from thesinglet excited state (fluorescence) is observed. Therefore, theinternal quantum efficiency (the ratio of generated photons to injectedcarriers) of a light-emitting element using a fluorescent material isassumed to have a theoretical limit of 25% based on the ratio of S* toT* that is 1:3.

In contrast, in a material that emits light from the triplet excitedstate (hereinafter referred to as a phosphorescent material), lightemission from the triplet excited state (phosphorescence) is observed.Since intersystem crossing easily occurs in a phosphorescent material,the internal quantum efficiency can be increased to 100% in theory. Thatis, a light-emitting element using a phosphorescent material can havehigher emission efficiency than a light-emitting element using afluorescent material. For this reason, light-emitting elements usingphosphorescent materials are now under active development in order toobtain highly efficient light-emitting elements (see Patent Document 1).

REFERENCE Patent Document [Patent Document 1] Japanese Published PatentApplication No. 2012-186461 SUMMARY OF THE INVENTION

The light-emitting element using a phosphorescent material can havehigher emission efficiency than a light-emitting element using afluorescent material. In contrast, the lifetime of the light-emittingelement using a phosphorescent material is in some cases shorter thanthat of the light-emitting element using a fluorescent material.Particularly in the light-emitting element using a phosphorescentmaterial whose emission wavelength is located on the shorter wavelengthside, that is, a phosphorescent material whose emission spectrum has apeak in a blue wavelength range, it is difficult to obtaincharacteristics which can sufficiently achieve both high efficiency anda long lifetime.

In view of the above-described problems, an object of one embodiment ofthe present invention is to provide a novel light-emitting element.Another object of one embodiment of the present invention is to providea novel light-emitting element which can achieve both high efficiencyand a long lifetime.

Another object of one embodiment of the present invention is to providea light-emitting device, an electronic device, and a lighting devicethat include the light-emitting element.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

According to one embodiment of the present invention, a light-emittingelement includes a light-emitting layer between a pair of electrodes.The light-emitting element includes a first light-emitting layer and asecond light-emitting layer. The first light-emitting layer includes afluorescent material. The second light-emitting layer includes aphosphorescent material. A difference in peak value between a firstemission spectrum of light from the first light-emitting layer and asecond emission spectrum of light from the second light-emitting layeris 30 nm or less.

In the above embodiment, emission color of light from the fluorescentmaterial is preferably the same as or similar to emission color of lightfrom the phosphorescent material. Moreover, in the above embodiment, thefirst emission spectrum and the second emission spectrum each preferablyhave the peak value in a blue wavelength range.

According to another embodiment of the present invention, alight-emitting element includes a light-emitting layer between a pair ofelectrodes. The light-emitting element includes a first EL layer and asecond EL layer. The first EL layer includes a first light-emittinglayer and a second light-emitting layer. The first light-emitting layerincludes a fluorescent material. The second light-emitting layerincludes a first phosphorescent material. The second EL layer includes athird light-emitting layer. The third light-emitting layer includes asecond phosphorescent material. A difference in peak value between afirst emission spectrum of light from the first light-emitting layer anda second emission spectrum of light from the second light-emitting layeris 30 nm or less.

In the above embodiment, emission color of light from the fluorescentmaterial is preferably the same as or similar to emission color of lightfrom the first phosphorescent material. Moreover, in the aboveembodiment, the first emission spectrum and the second emission spectrumeach preferably have the peak value in a blue wavelength range.

According to another embodiment of the present invention, alight-emitting element includes a light-emitting layer between a pair ofelectrodes. The light-emitting element includes a first EL layer and asecond EL layer. The first EL layer includes a first light-emittinglayer and a second light-emitting layer. The first light-emitting layerincludes a fluorescent material. The second light-emitting layerincludes a first phosphorescent material. The second EL layer includes athird light-emitting layer and a fourth light-emitting layer. The thirdlight-emitting layer includes a second phosphorescent material. Thefourth light-emitting layer includes a third phosphorescent material. Adifference in peak value between a first emission spectrum of light fromthe first light-emitting layer and a second emission spectrum of lightfrom the second light-emitting layer is 30 nm or less.

In the above embodiment, emission color of light from the fluorescentmaterial is preferably the same as or similar to emission color of lightfrom the first phosphorescent material. Moreover, in the aboveembodiment, the first emission spectrum and the second emission spectrumeach preferably have the peak value in a blue wavelength range.

According to another embodiment of the present invention, alight-emitting device includes the above-described light-emittingelement and a color filter. According to another embodiment of thepresent invention, an electronic device includes the above-describedlight-emitting element or the above-described light-emitting device, anda touch sensor. According to another embodiment of the presentinvention, a lighting device includes the above-described light-emittingelement or the above-described electronic device, and a housing.

According to one embodiment of the present invention, a novellight-emitting element can be provided. Alternatively, a novellight-emitting element which can achieve both high efficiency and a longlifetime can be provided. Alternatively, a light-emitting device, anelectronic device, and a lighting device that include the light-emittingelement can be provided.

Note that the descriptions of these effects do not disturb the existenceof other effects. One embodiment of the present invention does notnecessarily achieve all the effects. Other effects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic cross-sectional views illustrating alight-emitting element.

FIGS. 2A and 2B are schematic cross-sectional views illustrating alight-emitting element.

FIGS. 3A and 3B are schematic cross-sectional views illustrating alight-emitting element.

FIGS. 4A and 4B show characteristics of a light-emitting element 1 and alight-emitting element 2.

FIG. 5 shows emission spectra of the light-emitting element 1 and thelight-emitting element 2.

FIG. 6 shows luminance degradation of the light-emitting element 1 andthe light-emitting element 2.

FIG. 7 shows luminance degradation of the light-emitting element 1, thelight-emitting element 2, and a light-emitting element 3.

FIGS. 8A and 8B are a schematic cross-sectional view of a light-emittingelement and a diagram illustrating the correlation of energy levels in alight-emitting layer.

FIGS. 9A and 9B are a schematic cross-sectional view of a light-emittingelement and a diagram illustrating the correlation of energy levels in alight-emitting layer.

FIGS. 10A and 10B are a schematic cross-sectional view of alight-emitting element and a diagram illustrating the correlation ofenergy levels in a light-emitting layer.

FIG. 11 is a schematic cross-sectional view illustrating alight-emitting element.

FIG. 12 is a schematic cross-sectional view illustrating alight-emitting element.

FIG. 13 is a schematic cross-sectional view illustrating alight-emitting element.

FIGS. 14A and 14B are a block diagram and a circuit diagram illustratinga display device.

FIGS. 15A and 15B are each a circuit diagram illustrating a pixelcircuit of a display device.

FIGS. 16A and 16B are each a circuit diagram illustrating a pixelcircuit of a display device.

FIGS. 17A and 17B are perspective views of an example of a touch panel.

FIGS. 18A to 18C are cross-sectional views of examples of a displaypanel and a touch sensor.

FIGS. 19A and 19B are each a cross-sectional view illustrating anexample of a touch panel.

FIGS. 20A and 20B are a block diagram and a timing chart of a touchsensor.

FIG. 21 is a circuit diagram of a touch sensor.

FIG. 22 is a perspective view illustrating a display module.

FIGS. 23A to 23G illustrate electronic devices.

FIGS. 24A to 24C are a perspective view and cross-sectional viewsillustrating a light-emitting device.

FIGS. 25A to 25D are cross-sectional views illustrating a light-emittingdevice.

FIGS. 26A to 26C illustrate a lighting device and an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the drawings. Note that one embodiment of the presentinvention is not limited to the following description, and the modes anddetails thereof can be modified in various ways without departing fromthe spirit and scope of the present invention. Accordingly, oneembodiment of the present invention should not be interpreted as beinglimited to the content of the embodiments below.

Note that the position, the size, the range, or the like of eachstructure illustrated in drawings and the like is not accuratelyrepresented in some cases for easy understanding. Therefore, oneembodiment of the disclosed invention is not necessarily limited to theposition, the size, the range, or the like disclosed in the drawings andthe like.

The ordinal numbers such as “first” and “second” in this specificationand the like are used for convenience and do not denote the order ofsteps or the stacking order of layers. Therefore, for example,description can be made even when “first” is replaced with “second” or“third”, as appropriate. In addition, the ordinal numbers in thisspecification and the like are not necessarily the same as those whichspecify one embodiment of the present invention.

In order to describe structures of the invention with reference to thedrawings in this specification and the like, the same reference numeralsare used in common for the same portions in different drawings.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other. For example, the term “conductive layer”can be changed into the term “conductive film” in some cases. Also, theterm “insulating film” can be changed into the term “insulating layer”in some cases.

EMBODIMENT 1

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIGS. 1A and 1B,FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 4A and 4B, FIG. 5, FIG. 6, andFIG. 7.

<1-1. Structure 1 of Light-Emitting Element>

FIGS. 1A and B are schematic cross-sectional views of a light-emittingelement 100 of one embodiment of the present invention.

The light-emitting element 100 illustrated in FIG. 1A includes a firstlight-emitting layer 110 and a second light-emitting layer 112 between afirst electrode 104 and a second electrode 114. The light-emittingelement 100 in FIG. 1A further includes a hole-injection layer 131, ahole-transport layer 132, an electron-transport layer 133, and anelectron-injection layer 134, in addition to the first light-emittinglayer 110 and the second light-emitting layer 112.

More specifically, the light-emitting element 100 includes the firstelectrode 104 over a substrate 102, the hole-injection layer 131 overthe first electrode 104, the hole-transport layer 132 over thehole-injection layer 131, the first light-emitting layer 110 over thehole-transport layer 132, the second light-emitting layer 112 over thefirst light-emitting layer 110, the electron-transport layer 133 overthe second light-emitting layer 112, the electron-injection layer 134over the electron-transport layer 133, and the second electrode 114 overthe electron-injection layer 134.

Note that layers between a pair of electrodes (here, the hole-injectionlayer 131, the hole-transport layer 132, the first light-emitting layer110, the second light-emitting layer 112, the electron-transport layer133, and the electron-injection layer 134) are collectively referred toas an EL layer 108.

The first light-emitting layer 110 includes at least a fluorescentmaterial. Note that the first light-emitting layer 110 may includeanother material such as a host material or an assist material inaddition to the fluorescent material. For example, in the firstlight-emitting layer 110, the host material is present in the highestproportion by weight, and the fluorescent material is dispersed in thehost material. In the first light-emitting layer 110, it is preferablethat the S₁ level of the host material be higher than the S₁ level ofthe fluorescent material, and that the T₁ level of the host material belower than the T₁ level of the fluorescent material.

The second light-emitting layer 112 includes at least a phosphorescentmaterial. Note that the second light-emitting layer 112 may includeanother material such as a host material or an assist material inaddition to the phosphorescent material. For example, in the secondlight-emitting layer 112, the host material is present in the highestproportion by weight, and the phosphorescent material is dispersed inthe host material. In the second light-emitting layer 112, it ispreferable that the T₁ level of the host material be higher than the T₁level of the fluorescent material.

Note that a material whose emission spectrum has a peak in a bluewavelength range is particularly preferable for the fluorescent materialof the first light-emitting layer 110. Moreover, a material whoseemission spectrum has a peak in a blue wavelength range is particularlypreferable for the phosphorescent material of the second light-emittinglayer 112. The blue wavelength range is preferably 400 nm or more and500 nm or less and further preferably 420 nm or more and 480 nm or less.

A difference in peak value between a first emission spectrum of lightfrom the first light-emitting layer 110 and a second emission spectrumof light from the second light-emitting layer 112 is 30 nm or less,preferably 25 nm or less and further preferably 20 nm or less. In otherwords, the emission color of light from the first light-emitting layer110 is preferably the same as or similar to the emission color of lightfrom the second light-emitting layer 112.

For example, when a blue fluorescent material is used for the firstlight-emitting layer 110 and a blue phosphorescent material is used forthe second light-emitting layer 112, the light-emitting element 100 canachieve both high efficiency and a long lifetime.

Although, in FIG. 1A, the first light-emitting layer 110 and the secondlight-emitting layer 112 are provided in contact with each other, forexample, one embodiment of the present invention is not limited to sucha structure. As illustrated in FIG. 1B, a buffer layer 140 may beprovided between the first light-emitting layer 110 and the secondlight-emitting layer 112, for example.

The buffer layer 140 is provided to prevent energy transfer by theDexter mechanism (particularly triplet energy transfer) from the hostmaterial in an excited state or the phosphorescent material in anexcited state which is generated in the second light-emitting layer 112to the host material or the fluorescent material in the firstlight-emitting layer 110. Thus, the thickness of the buffer layer 140may be several nanometers. Specifically, the thickness of the bufferlayer 140 may be greater than or equal to 0.1 nm and less than or equalto 20 nm, greater than or equal to 1 nm and less than or equal to 10 nm,or greater than or equal to 1 nm and less than or equal to 5 nm.

The buffer layer 140 may include a single material or both ahole-transport material and an electron-transport material. In the caseof a single material, a bipolar material may be used. The bipolarmaterial here refers to a material in which the ratio between theelectron mobility and the hole mobility is 100 or less. As a materialincluded in the buffer layer 140, a hole-transport material, anelectron-transport material, or the like can be used. The hole-transportmaterial or the electron-transport material will be described later. Thematerial included in the buffer layer 140 is preferably formed of thesame material as the host material of the second light-emitting layer112. This facilitates the manufacture of the light-emitting element 100and reduces the driving voltage of the light-emitting element 100.

For example, when the buffer layer 140 is formed of the same materialsas the host material and the assist material of the secondlight-emitting layer 112, the first light-emitting layer 110 and thesecond light-emitting layer 112 are stacked with each other while thelayer (the buffer layer 140) not including the phosphorescent materialof the second light-emitting layer 112 is provided therebetween. In thecase of such a structure, depending on using or not using thephosphorescent material, the second light-emitting layer 112 or thebuffer layer 140 can be deposited. In other words, the buffer layer 140includes a region not including the phosphorescent material while thesecond light-emitting layer 112 includes a region including thephosphorescent material.

Note that a material included in the buffer layer 140 may have a higherT₁ level than the host material of the second light-emitting layer 112.

For example, in the case where the buffer layer 140 includes ahole-transport material and an electron-transport material, a carrierrecombination region can be adjusted by adjusting the mixture ratio ofthe hole-transport material and the electron-transport material. Forexample, in the case where the first electrode 104 and the secondelectrode 114 serve as an anode and a cathode, respectively, the carrierrecombination region can be shifted from the first electrode 104 side tothe second electrode 114 side by increasing the proportion of thehole-transport material in the buffer layer 140. As a result, thecontribution of the second light-emitting layer 112 to light emissioncan be increased. In contrast, by increasing the proportion of theelectron-transport material in the buffer layer 140, the carrierrecombination region can be shifted from the second electrode 114 sideto the first electrode 104 side, so that the contribution of the firstlight-emitting layer 110 to light emission can be increased.

The hole-transport material and the electron-transport material may forman exciplex in the buffer layer 140, which effectively prevents excitondiffusion. Specifically, energy transfer from the host material in anexcited state or the phosphorescent material in an excited state of thesecond light-emitting layer 112 to the host material or the fluorescentmaterial of the first light-emitting layer 110 can be prevented.

Although, in FIGS. 1A and 1B, the first light-emitting layer 110 islocated on the first electrode 104 side and the second light-emittinglayer 112 is located on the second electrode 114 side, for example, oneembodiment of the present invention is not limited to such a structure.For example, as illustrated in FIGS. 2A and 2B, the first light-emittinglayer 110 may be located on the second electrode 114 side and the secondlight-emitting layer 112 may be located on the first electrode 104 side.

<1-2. Characteristics and Luminance Degradation of Light-EmittingElements>

Here, characteristics and luminance degradation of a light-emittingelement including a fluorescent material and a light-emitting elementincluding a phosphorescent material are described. First, thelight-emitting element including a fluorescent material (alight-emitting element 1) and the light-emitting element including aphosphorescent material (a light-emitting element 2) were fabricated andtheir characteristics and luminance degradation were evaluated.

Schematic cross-sectional views of the light-emitting elements 1 and 2are illustrated in FIGS. 3A and 3B, the detailed structures of thelight-emitting elements 1 and 2 are shown in Table 1, and structures andabbreviations of compounds used here are given below. Note that FIG. 3Ais the schematic cross-sectional view of the light-emitting element 1and FIG. 3B is the schematic cross-sectional view of the light-emittingelement 2.

TABLE 1 Reference Thickness Weight Layer numeral (nm) Material ratioLight- Second electrode 114 200 Al — emitting Electron-injection layer134(2) 1 LiF — element 1 134(1) 15 Bphen — Electron-transport layer 13310 cgDBCzPA — Light-emitting layer 116 20 cgDBCzPA:1,6mMemFLPAPrn 1:0.03Hole-transport layer 132 20 PCPPn — Hole-injection layer 131 20PCPPn:MoOx 2:1 First electrode 104 70 ITSO — Light- Second electrode 114200 Al — emitting Electron-injection layer 134(2) 1 LiF — element 2134(1) 15 Bphen — Electron-transport layer 133 10 3,5DCzPPy —Light-emitting layer 116(2) 10 3,5DCzPPy:Ir(mpptz-diPrp)₃ 1:0.06 116(1)30 PCCP:3,5DCzPPy:Ir(mpptz-diPrp)₃ 1:0.3:0.06 Hole-transport layer 13220 PCCP — Hole-injection layer 131 20 DBT3P-II:MoOx 2:1 First electrode104 70 ITSO —

<1-3. Method for Fabricating Light-Emitting Element 1>

As the first electrode 104, an oxide containing silicon, indium, and tin(abbreviation: ITSO) was deposited over the substrate 102 by asputtering method. Note that the thickness of the first electrode 104was 70 nm, and the area of the first electrode 104 was 4 mm² (2 mm×2mm).

Next, as pretreatment of evaporation of an organic compound layer, thefirst electrode 104 side of the substrate 102 was washed with water,baking was performed at 200° C. for one hour, and then UV ozonetreatment was performed on a surface of the first electrode 104 for 370seconds.

After that, the substrate 102 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to about 1×10⁻⁴ Pa, andwas subjected to vacuum baking at 170° C. for 60 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate 102was cooled down for about 30 minutes.

Next, the substrate 102 was fixed to a holder in the vacuum evaporationapparatus so that a side on which the first electrode 104 was providedfaced downward. In this embodiment, the hole-injection layer 131, thehole-transport layer 132, a light-emitting layer 116, theelectron-transport layer 133, an electron-injection layer 134(1), anelectron-injection layer 134(2), and the second electrode 114 weresequentially formed by a vacuum evaporation method.

First, the pressure in the vacuum evaporation apparatus was reduced toabout 1×10⁴ Pa. Then, 3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole(abbreviation: PCPPn) and molybdenum oxide were deposited byco-evaporation at a weight ratio of 2·1 (=PcPPn: molybdenum oxide),whereby the hole-injection layer 131 was formed over the first electrode104. Note that the thickness of the hole-injection layer 131 was 20 nm.

Then, the hole-transport layer 132 was formed over the hole-injectionlayer 131. As the hole-transport layer 132, PCPPn was evaporated. Notethat the thickness of the hole-transport layer 132 was 20 nm.

Next, the light-emitting layer 116 was formed over the hole-transportlayer 132. As the light-emitting layer 116,7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[cg]carbazole (abbreviation:cgDBCzPA) andN,N-bis(3-methylphenyl)-N,N-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) were deposited by co-evaporation at aweight ratio of 1:0.03 (=cgDBCzPA:1,6mMemFLPAPrn). Note that thethickness of the light-emitting layer 116 was 20 nm. In thelight-emitting layer 116, cgDBCzPA serves as a host material and1,6mMemFLPAPrn serves as a guest material.

After that, as the electron-transport layer 133, cgDBCzPA was depositedover the light-emitting layer 116 by evaporation to a thickness of 10nm. Then, as the electron-injection layer 134(1), bathophenanthroline(abbreviation: Bphen) was deposited over the electron-transport layer133 by evaporation to a thickness of 15 nm. Then, as theelectron-injection layer 134(2), lithium fluoride (LiF) was depositedover the electron-injection layer 134(1) by evaporation to a thicknessof 1 nm.

Then, as the second electrode 114, aluminum (Al) was deposited over theelectron-injection layer 134(2) by evaporation. Note that the thicknessof the second electrode 114 was 200 nm.

The light-emitting element over the substrate 102 fabricated asdescribed above was sealed by being bonded to a sealing substrate (notillustrated) in a glove box in a nitrogen atmosphere so as not to beexposed to the air (specifically, a sealant was applied to surround theelement, and irradiation with ultraviolet light having a wavelength of365 nm at 6 J/cm² and heat treatment at 80° C. for one hour wereperformed for sealing).

Through the above process, the light-emitting element 1 was fabricated.

<1-4. Method for Fabricating Light-Emitting Element 2>

First, as the first electrode 104, ITSO was deposited over the substrate102 by a sputtering method. Note that the thickness of the firstelectrode 104 was 70 nm, and the area of the first electrode 104 was 4mm² (2 mm×2 mm).

Next, as pretreatment of evaporation of an organic compound layer, thefirst electrode 104 side of the substrate 102 was washed with water,baking was performed at 200° C. for one hour, and then UV ozonetreatment was performed on a surface of the first electrode 104 for 370seconds.

After that, the substrate 102 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to about 1×10⁻⁴ Pa, andwas subjected to vacuum baking at 170° C. for 60 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate 102was cooled down for about 30 minutes.

Next, the substrate 102 was fixed to a holder in the vacuum evaporationapparatus so that a side on which the first electrode 104 was providedfaced downward. In this embodiment, the hole-injection layer 131, thehole-transport layer 132, a light-emitting layer 116(1), alight-emitting layer 116(2), the electron-transport layer 133, theelectron-injection layer 134(1), the electron-injection layer 134(2),and the second electrode 114 were sequentially formed by a vacuumevaporation method.

First, after reducing the pressure of the vacuum evaporation apparatusto 1×10⁻⁴ Pa, 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene)(abbreviation: DBT3P-II) and molybdenum oxide were deposited byco-evaporation at a weight ratio of 2:1 (=DBT3P-II: molybdenum oxide),whereby the hole-injection layer 131 was formed over the first electrode104. Note that the thickness of the hole-injection layer 131 was 20 nm.

Then, the hole-transport layer 132 was formed over the hole-injectionlayer 131. As the hole-transport layer 132,9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP)was deposited by evaporation. Note that the thickness of thehole-transport layer 132 was 20 nm.

Next, the light-emitting layer 116(1) was formed over the hole-transportlayer 132. As the light-emitting layer 116(1), PCCP,3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 3,5DCzPPy),andtris{2-[5-(2-methylphenyl)-4-(2,6-diisopropylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(TT)(abbreviation: Ir(mpptz-diPrp)₃) were deposited by co-evaporation at aweight ratio of 1:0.3:0.06 (=. PCCP:3,5DCzPPy:Ir(mpptz-diPrp)₃). Notethat the thickness of the light-emitting layer 116(1) was 30 nm. Notethat in the light-emitting layer 116(1), PCCP serves as a host material,3,5DCzPPy serves as an assist material, and Ir(mpptz-diPrp)₃ serves as aguest material.

Next, the light-emitting layer 116(2) was formed over the light-emittinglayer 116(1). As the light-emitting layer 116(2), 3,5DCzPPy andIr(mpptz-diPrp)₃ were deposited by co-evaporation at a weight ratio of1:0.06 (=3,5DCzPPy:Ir(mpptz-diPrp)₃). Note that the thickness of thelight-emitting layer 116(2) was 10 nm. Note that in the light-emittinglayer 116(2), 3,5DCzPPy serves as a host material, and Ir(mpptz-diPrp),serves as a guest material.

After that, as the electron-transport layer 133, 3,5DCzPPy was depositedover the light-emitting layer 116(2) by evaporation to a thickness of 10nm. Then, as the electron-injection layer 134(1), Bphen was depositedover the electron-transport layer 133 by evaporation to a thickness of15 nm. Then, as the electron-injection layer 134(2), LiF was depositedover the electron-injection layer 134(1) by evaporation to a thicknessof 1 nm.

Then, as the second electrode 114, aluminum (Al) was deposited over theelectron-injection layer 134(2) by evaporation. Note that the thicknessof the second electrode 114 was 200 nm.

The light-emitting element over the substrate 102 fabricated asdescribed above was sealed by being bonded to a sealing substrate (notillustrated) in a glove box in a nitrogen atmosphere so as not to beexposed to the air. Note that the sealing method was the same as that ofthe light-emitting element 1.

Through the above process, the light-emitting element 2 was fabricated.

Note that in all the above evaporation steps for the light-emittingelements 1 and 2, a resistive heating method was used as an evaporationmethod.

<1-5. Characteristics of Light-Emitting Elements 1 and 2>

Next, characteristics of the fabricated light-emitting elements 1 and 2were measured. Note that the light-emitting elements 1 and 2 weremeasured at room temperature (in an atmosphere kept at 25° C.).

FIG. 4A shows the current efficiency vs. current density characteristicsof the light-emitting elements 1 and 2. FIG. 4B shows the externalquantum efficiency vs. current density characteristics of thelight-emitting elements 1 and 2. The major element characteristics ofthe light-emitting elements 1 and 2 when the current densities thereofare 5 mA/cm² are shown in Table 2.

TABLE 2 Current density CE EQE Luminance (mA/cm²) (cd/A) (%) (cd/m²)CIE(x) CIE(y) Light-emitting 5 14 12  700 0.14 0.16 element 1Light-emitting 5 60 28 3000 0.17 0.38 element 2

Note that in Table 2, CE represents current efficiency, EQE representsexternal quantum efficiency, and CIB represents chromaticity(chromaticity coordinates in the CIE 1976 chromaticity system).

As shown in FIGS. 4A and 4B and Table 2, the current efficiency and theexternal quantum efficiency of the light-emitting element including aphosphorescent material (light-emitting element 2) are 4.3 times and 2.3times, respectively, as high as those of the light-emitting elementincluding a fluorescent material (light-emitting element 1). Thelight-emitting element including a phosphorescent material(light-emitting element 2) thus has higher emission efficiency than thelight-emitting element including a fluorescent material (light-emittingelement 1).

FIG. 5 shows emission spectra when a current at a current density of 2.5mA/cm² was supplied to the light-emitting elements 1 and 2.

As shown in FIG. 5 and Table 2, emission spectra of the light-emittingelements 1 and 2 each have a peak in a blue wavelength range. In otherwords, the emission color of light from the light-emitting layer 1 isthe same as or similar to the emission color of light from thelight-emitting layer 2. Note that the peak of the emission spectrum ofthe light-emitting element 1 was 465 nm, and the peak of the emissionspectrum of the light-emitting element 2 was 476 nm. In other words, thefluorescent material exhibits emission at a shorter wavelength than thephosphorescent material and the difference between the emission peaks is11 nm.

<1-6. Luminance Degradation of Light-Emitting Elements 1 and 2>

Next, luminance degradation of the light-emitting elements 1 and 2 wasevaluated. As a method for evaluating the luminance degradation, thelight-emitting element 1 and the light-emitting element 2 were subjectedto constant driving at a current density of 36.3 mA/cm² (initialluminance of 4930 cd/m²) and a current density of 2.03 mA/cm² (initialluminance of 1270 cd/m²), respectively.

The evaluation result of the luminance degradation is shown in FIG. 6.In FIG. 6, the vertical axis represents normalized luminance (%) withthe initial luminance of 100%, and the horizontal axis representsdriving time (h) of the element.

As shown in FIG. 6, the normalized luminance of the light-emittingelement including a fluorescent material (light-emitting element 1) isdegrading more slowly than that of the light-emitting element includinga phosphorescent material (light-emitting element 2). In other words,the light-emitting element including a fluorescent material(light-emitting element 1) has a longer lifetime than the light-emittingelement including a phosphorescent material (light-emitting element 2).

Note that in FIG. 6, the light-emitting element including a fluorescentmaterial (light-emitting element 1) and the light-emitting elementincluding a phosphorescent material (light-emitting element 2) are eachdriven at a different current density, i.e., a different luminance;however, in the case of being driven at the same luminance, thelight-emitting element including a fluorescent material (light-emittingelement 1) has a much longer lifetime than the light-emitting elementincluding a phosphorescent material (light-emitting element 2).

As described above, the light-emitting element including aphosphorescent material has higher efficiency but a shorter lifetimethan the light-emitting element including a fluorescent material. Incontrast, the light-emitting element including a fluorescentlight-emitting element has lower efficiency but a longer lifetime thanthe light-emitting element including a phosphorescent material.

In view of this, with a stack of a light-emitting layer including afluorescent material and a light-emitting layer including aphosphorescent material as the light-emitting element of one embodimentof the present invention, a light-emitting element with high efficiencyand a long lifetime can be achieved. In addition, the emission color oflight from the fluorescent material is preferably the same as or similarto the emission color of light from the phosphorescent material. Forexample, a difference in peak value between an emission spectrum oflight from the light-emitting layer including a fluorescent material andan emission spectrum of light from the light-emitting layer including aphosphorescent material is 30 nm or less, preferably 20 nm or less, andfurther preferably 15 nm or less. It is preferable in terms ofreliability that the emission wavelength of the fluorescent material belocated on the shorter wavelength side than the phosphorescent material.A difference in peak value of the emission spectrum between thematerials is preferably 5 nm or more.

<1-7. Calculation Results of Initial Characteristics>

Here, initial characteristics of the light-emitting element of oneembodiment of the present invention, i.e., a light-emitting element inwhich a light-emitting layer including a fluorescent material and alight-emitting layer including a phosphorescent material are stacked(hereinafter referred to as a light-emitting element 3) was calculated.

Note that the calculation was made on the basis of the following threehypotheses:

(1) the ratio of excitons generated in the light-emitting layerincluding a fluorescent material to excitons generated in thelight-emitting layer including a phosphorescent material was 0.8 to 0.2;

(2) the lifetime of a light-emitting element including the fluorescentmaterial was inversely proportional to the initial luminance to thepower of 1.8, and the lifetime of a light-emitting element including thephosphorescent material was inversely proportional to the initialluminance to the power of 2.0 (note that in one embodiment of thepresent invention, accelerating factor for luminance of the fluorescentmaterial is preferably higher than accelerating factor for luminance ofthe phosphorescent material because such accelerating factor forluminance is higher in a phosphorescent material in many cases); and

(3) the shapes of their luminance degradation curves were the same anddid not depend on their initial characteristics.

Note that the ratio of generated excitons is not limited to the aboveand can be set to an optimal ratio as appropriate by a practitioner.

Calculation results are shown in Table 3. Note that Table 3 showscalculation results of major element characteristics of thelight-emitting element 3.

TABLE 3 Current Fluorescent Phosphorescent density CE EQE Luminanceluminance luminance (mA/cm²) (cd/A) (%) (cd/m²) (cd/m²) (cd/m²) CIE(x)CIE(y) Light-emitting 5 23.2 15.2 1160 560 600 0.15 0.23 element 3

The element characteristics of the light-emitting element 3 werecalculated on the basis of the element characteristics of thelight-emitting elements 1 and 2 in Table 2. Efficiency (currentefficiency and external quantum efficiency) of the light-emittingelement 3 can be obtained from the following formula 1.

Efficiency of light-emitting element 3=(efficiency of light-emittingelement 1)×0.8+(efficiency of light-emitting element 2)×0.2  [Formula 1]

Note that the luminance of the light-emitting element 3 can be obtainedby similar calculation. That is, the luminance of the light-emittingelement 3 in Table 3 is a value (1160 cd/m²) obtained by adding aluminance of 560 cd/m² which is obtained by multiplying the luminance ofthe light-emitting element 1 (700 cd/m²) by 0.8 to a luminance of 600cd/m² which is obtained by multiplying the luminance of thelight-emitting element 2 (3000 cd/m²) by 0.2. Note that in Table 3, theluminance of the light-emitting layer including a fluorescent materialand the luminance of the light-emitting layer including a phosphorescentmaterial are represented as fluorescent luminance and phosphorescentluminance, respectively.

According to the results shown in Table 2 and Table 3, the currentefficiency and the external quantum efficiency of the light-emittingelement 3 are 1.7 times and 1.3 times, respectively, as high as those ofthe light-emitting element 1.

<1-8. Calculation Results of Luminance Degradation>

Next, degradation curves when the light-emitting elements 1 to 3 areeach driven at a luminance of 1160 cd/m² were calculated. Calculationresults are shown in FIG. 7.

The luminance degradation curve of the light-emitting element 1 in FIG.7 was (4930/1160)^(1.8), times of that in FIG. 6, and the luminancedegradation curve of the light-emitting element 2 in FIG. 7 was(1270/1160)² times of that in FIG. 6. A luminance degradation curvewhich was obtained by assuming that the light-emitting element 1 wasdriven at a luminance of 560 cd/m² and a luminance degradation curvewhich was obtained by assuming that the light-emitting element 2 wasdriven at a luminance of 600 cd/m² were calculated by a method similarto the above, and the two luminance degradation curves were addedtogether to obtain the luminance degradation curve of the light-emittingelement 3.

As shown in FIG. 7, the driving times required for 80% of initialluminance of the light-emitting elements, i.e., 928 cd/cm² are about6000 hours in the light-emitting element 1, about 220 hours in thelight-emitting element 2, and about 2000 hours in the light-emittingelement 3. In other words, the lifetime of the light-emitting element 3is almost one-third of that of the light-emitting element 1 and almost 9times as long as that of the light-emitting element 2.

Here, the results of the cases in which the element characteristics ofthe light-emitting element 1 and the light-emitting element 2 werenormalized by the light-emitting element 3 (normalized CE, normalizedEQE, and normalized lifetime) are shown in Table 4. Note that in Table4, when initial luminance is assumed to be 100V, normalized lifetime(LT80) corresponds to time required for 80% of luminance of the initialluminance.

TABLE 4 Normalized Normalized Normalized lifetime CE EQE (LT80) CIE(x)CIE(y) Light- 0.60 0.79 3.0 0.14 0.16 emitting element 1 Light- 2.6 1.840.11 0.17 0.38 emitting element 2 Light- 1 1 1 0.15 0.23 emittingelement 3

As shown in Table 4, the light-emitting element 3 of one embodiment ofthe present invention has higher efficiency than the light-emittingelement 1 and has a longer lifetime than the light-emitting element 2.Specifically, the lifetime of the light-emitting element 3 is 9 times aslong as that of the light-emitting element 2 including only aphosphorescent material, and the light-emitting element 3 has a higheffect on a long lifetime despite a mere reduction in emissionefficiency by about 60% in CE and about 45% in EQE as compared with thelight-emitting element 2. This is an effect that cannot be expected, butthe factor of the effect is probably due to a difference in luminance(current) accelerating factor between phosphorescence and fluorescenceand a difference in current efficiency, which affect each other.Moreover, the emission efficiency of the light-emitting element 3 ishigher than at least the emission efficiency of the light-emittingelement 1 including only a fluorescent material. That is, thelight-emitting element of one embodiment of the present invention cantheoretically obtain higher emission efficiency than the light-emittingelement including only a fluorescent material while maintaining alifetime that is necessary to fabricate a product even with a bluephosphorescent material.

Note that the ratio of excitons generated in a fluorescentlight-emitting layer to excitons generated in a phosphorescentlight-emitting layer is preferably in the range of 0.9:0.1 to 0.5:0.5from the these calculation results.

<1-9. Description of Components of Light-Emitting Elements>

Next, details of components of the light-emitting element 100 in FIGS.1A and 1B and FIGS. 2A and 2B will be described.

[Substrate]

The substrate 102 is used as a support of the light-emitting element100. For the substrate 102, glass, quartz, plastic, or the like can beused, for example. Alternatively, a flexible substrate can be used. Aflexible substrate is a substrate that can be bent; examples of theflexible substrate include a plastic substrate made of a polycarbonate,a polyarylate, or a polyethersulfone. A film (made of polypropylene, apolyester, poly(vinyl fluoride), poly(vinyl chloride), or the like), aninorganic vapor-deposited film, or the like can be used.

The substrate may be formed with any other material that can serve as asupport in a fabrication process of the light-emitting element 100. Thelight-emitting element 100 can be formed using a variety of substrates,for example. The type of the substrate is not limited to a certain type.As the substrate, a semiconductor substrate (e.g., a single crystalsubstrate or a silicon substrate), an SOI substrate, a glass substrate,a quartz substrate, a plastic substrate, a metal substrate, a stainlesssteel substrate, a substrate including stainless steel foil, a tungstensubstrate, a substrate including tungsten foil, a flexible substrate, anattachment film, paper including a fibrous material, a base materialfilm, or the like can be used, for example. Examples of a glasssubstrate include a barium borosilicate glass substrate, analuminoborosilicate glass substrate, and a soda lime glass substrate.Examples of the flexible substrate, the attachment film, the base film,and the like are substrates of plastics typified by polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone(PES), and polytetrafluoroethylene (PTFE). Another example is asynthetic resin such as acrylic. Furthermore, polypropylene, polyester,polyvinyl fluoride, and polyvinyl chloride can be given as examples.Other examples are polyamide, polyimide, aramid, epoxy, an inorganicvapor deposition film, paper, and the like.

Alternatively, a flexible substrate may be used as the substrate, andthe light-emitting element 100 may be provided directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thesubstrate and the light-emitting element 100. The separation layer canbe used when part or the whole of the light-emitting element 100 formedover the separation layer is completed, separated from the substrate,and transferred to another substrate. In such a case, the light-emittingelement 100 can be transferred to a substrate having low heat resistanceor a flexible substrate as well. For the above separation layer, a stackincluding inorganic films, which are a tungsten film and a silicon oxidefilm, or an organic resin film of polyimide or the like formed over asubstrate can be used, for example.

In other words, after the light-emitting element 100 is formed using asubstrate, the light-emitting element 100 may be transferred to anothersubstrate. Examples of a substrate to which the light-emitting element100 is transferred include, in addition to the above-describedsubstrates, a paper substrate, a cellophane substrate, an aramid filmsubstrate, a polyimide film substrate, a stone substrate, a woodsubstrate, a cloth substrate (including a natural fiber (e.g., silk,cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, orpolyester), a regenerated fiber (e.g., acetate, cupra, rayon, orregenerated polyester), or the like), a leather substrate, and a rubbersubstrate. By using such a substrate, the light-emitting element 100with high durability, the light-emitting element 100 with high heatresistance, the light-emitting element 100 that is lightweight, or thelight-emitting element 100 that is thin can be obtained.

[Pair of Electrodes]

As the first electrode 104 and the second electrode 114, a metal, analloy, an electrically conductive compound, a mixture thereof, and thelike can be used. Specific examples include an oxide containing indiumand tin (typically, indium tin oxide (ITO)), an oxide containingsilicon, indium, and tin (ITSO), an oxide containing indium, zinc,tungsten, and zinc, gold (Au), platinum (Pt), nickel (Ni), tungsten (W),chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu),palladium (Pd), and titanium (Ti). In addition, any of the followingmaterials can be used: elements that belong to Group 1 or Group 2 of theperiodic table, that is, alkali metals such as lithium (Li) and cesium(Cs) or alkaline earth metals such as calcium (Ca) and strontium (Sr),magnesium (Mg), and alloys containing at least one of the metals (e.g.,Mg—Ag and Al—Li); rare earth metals such as europium (Eu) and ytterbium(Yb), and alloys containing at least one of the metals; and graphene.The first electrode 104 and the second electrode 114 can be formed by,for example, a sputtering method, an evaporation method (including avacuum evaporation method), or the like.

One or both of the first electrode 104 and the second electrode 114 havelight-transmitting properties so that light emission from the EL layer108 can be extracted to the outside.

[First Light-Emitting Layer]

A material whose emission spectrum has a peak in a blue wavelength rangeis preferable for the fluorescent material of the first light-emittinglayer 110. However, the fluorescent material of the first light-emittinglayer 110 is not limited thereto, and a material whose emission spectrumhas a peak in a green, yellow, or red wavelength range may be used.

Examples of the fluorescent material of the first light-emitting layer110 include a pyrene derivative, an anthracene derivative, atriphenylene derivative, a fluorene derivative, a carbazole derivative,a dibenzothiophene derivative, a dibenzofuran derivative, adibenzoquinoxaline derivative, a quinoxaline derivative, a pyridinederivative, a pyrimidine derivative, a phenanthrene derivative, and anaphthalene derivative. A pyrene derivative is particularly preferablebecause it has a high emission quantum yield. Specific examples of thepyrene derivative includeN,N′-bis(3-methylphenyl)-N,N-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn),N,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N-bis(dibenzofuran-2-yl)-N,N-diphenylpyrene-1,6-diamine (abbreviation:1,6FrAPrn), andN,N-bis(dibenzothiophen-2-yl)-N,N-diphenylpyrene-1,6-diamine(abbreviation: 1,6ThAPrn).

An anthracene derivative or a tetracene derivative is preferably used asthe host material of the first light-emitting layer 110. This is becausethese derivatives each have a high S₁ level and a low T₁ level. Specificexamples include 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: PCzPA), 3-[4-(I-naphthyl)-phenyl]-9-phenyl-9H-carbazole(abbreviation: PCPN), 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole(abbreviation: CzPA),7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA),6-[3-(9,10-diphenyl-2-anthryl)phenyl]-benzo[b]naphtho[1,2-d]furan(abbreviation: 2mBnfPPA), and9-phenyl-10-{4-(9-phenyl-9H-fluoren-9-yl)biphenyl-4′-yl}anthracene(abbreviation: FLPPA). Besides, 5,12-diphenyltetracene,5,12-bis(biphenyl-2-yl)tetracene, and the like can be given.

[Second Light-Emitting Layer]

A material whose emission spectrum has a peak in a blue wavelength rangeis preferable for the phosphorescent material of the secondlight-emitting layer 112. However, the phosphorescent material of thesecond light-emitting layer 112 is not limited thereto, and a materialwhose emission spectrum has a peak in a green, yellow, or red wavelengthrange may be used.

As the phosphorescent material of the second light-emitting layer 112,an iridium-, rhodium-, or platinum-based organometallic complex or metalcomplex can be used; in particular, an organoiridium complex such as aniridium-based ortho-metalated complex is preferable. As anortho-metalated ligand, a 4H-triazole ligand, a 1H-triazole ligand, animidazole ligand, a triazole ligand, a pyridine ligand, a pyrimidineligand, a pyrazine ligand, a triazine ligand, a quinoline ligand, anisoquinoline ligand, or the like can be given. As the metal complex., aplatinum complex having a porphyrin ligand or the like can be given.

As specific examples of the organoiridium complex, iridium complexeshaving a triazole ligand, such astris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-xC}iridium(III) (abbreviation: Ir(mpptz-dmp)₃),tris{2-[5-(2-methylphenyl)-4-(2,6-diisopropylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}indium(III)(abbreviation: Ir(mpptz-diPrp)₃),tris(2-[4-{1-adamantyl)-3-methyl-4H-1,2,4-triazol-5-yl-N]phenyl-κC}iridium(III)(abbreviation: Ir(Mptz-Adm1)₃),tris{2-[4-(2-adamantyl)-3-methyl-4H-1,2,4-triazol-5-yl-κN]phenyl-κC}iridium(III)(abbreviation: Ir(Mptz-Adm2)₃), andtris{2-[4-(2-norbornyl)-3-methyl-4H-1,2,4-triazol-5-yl-κN]phenyl-κC}iridium(III)(abbreviation: Ir(Mptz-Nb)₃), are preferable as a blue phosphorescentmaterial. Moreover, an iridium complex having an imidazole ligand, suchastris(3-(2,4,6-trimethylphenyl)-4H-imidazol-3-yl-N2)phenyl-κCirridium(III)(abbreviation: Ir(tmppim)₃) ortris[1-(3,5-diisopropylphenyl)-2-phenyl-1H-imidazol-C2,N]iridium(III)(abbreviation: Ir(biprpim)₃) can also be used as a blue phosphorescentmaterial.

Examples of the host material of the second light-emitting layer 112include a zinc- or aluminum-based metal complex, an oxadiazolederivative, a triazole derivative, a benzimidazole derivative, aquinoxaline derivative, a dibenzoquinoxaline derivative, adibenzothiophene derivative, a dibenzofuran derivative, a pyrimidinederivative, a triazine derivative, a pyridine derivative, a bipyridinederivative, and a phenanthroline derivative. Other examples are anaromatic amine and a carbazole derivative.

As the assist material of the second light-emitting layer 112, asubstance which can form an exciplex together with the host material ispreferably used. In this case, it is preferable that the host material,the assist material, and the phosphorescent material be selected suchthat the emission peak of the exciplex overlaps with an absorption band,specifically an absorption band on the longest wavelength side, of atriplet metal to ligand charge transfer (MLCT) transition of thephosphorescent material. This makes it possible to provide alight-emitting element with drastically improved emission efficiency.However, a material exhibiting thermally activated delayed fluorescence(TADF) may be used instead of the phosphorescent material. This isbecause the behavior of the TADF material in elements is similar to thatof a phosphorescent material because emission energy of the TADFmaterial is substantially close to the triplet excitation energy (e.g.,in the case of using a blue TADF material, triplet excitation energy ofa peripheral material such as the host material needs to be as high asthat in the case of using a blue phosphorescent material). In the caseof using the TADF material, it is preferable that an absorption band onthe longest wavelength side be an absorption band of a singlet. Notethat the TADF material is a substance that can up-convert a tripletexcited state into a singlet excited state (i.e., reverse intersystemcrossing is possible) using a little thermal energy and efficientlyexhibits light emission (fluorescence) from the singlet excited state.The TADF is efficiently obtained under the condition where thedifference in energy between the triplet excited level and the singletexcited level is more than 0 eV and less than or equal to 0.2 eV,preferably more than 0 eV and less than or equal to 0.1 eV.

[Hole-Injection Layer and Hole-Transport Layer]

The hole-injection layer 131 is a layer that injects holes into thefirst light-emitting layer 110 through the hole-transport layer 132 witha high hole-transport property and includes a hole-transport materialand an acceptor material. When a hole-transport material and an acceptormaterial are included, electrons are extracted from the hole-transportmaterial by the acceptor substance to generate holes, and the holes areinjected into the first light-emitting layer 110 through thehole-transport layer 132. The hole-injection layer 131 may have astructure in which a hole-transport material and an acceptor materialare stacked. Note that the hole-transport layer 132 is formed using ahole-transport material.

Specific examples of the hole-transport materials used for thehole-injection layer 131 and the hole-transport layer 132 includearomatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N′-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB);3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1);3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2); and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1). Alternatively, the following carbazolederivatives can be used: 4,4′-di(N-carbazolyl)biphenyl (abbreviation:CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),and 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation:CzPA). These materials described here are mainly materials each having ahole mobility of greater than or equal to 1×10⁻⁶ cm²/Vs. Note that anyother material may be used as long as the material has a hole-transportproperty higher than an electron-transport property.

A high molecular compound such as poly(N-vinylcarbazole) (abbreviation:PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD) can also be used.

Examples of the acceptor material used in the hole-injection layer 131include compounds having an electron-withdrawing group (a halogen groupor a cyano group) such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT-CN). Inparticular, a compound in which electron-withdrawing groups are bondedto a condensed aromatic ring having a plurality of hetero atoms, likeHAT-CN, is thermally stable and preferable. Furthermore, the examplesalso include transition metal oxides. Moreover, the examples includeoxides of metals belonging to Groups 4 to 8 of the periodic table.Specifically, it is preferable to use vanadium oxide, niobium oxide,tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide,manganese oxide, and rhenium oxide because of their high electronaccepting properties. Among them, molybdenum oxide is particularlypreferable because of its stability in the atmosphere, low hygroscopicproperty, and easiness of handling.

Note that the hole-injection layer 131 may be formed of theabove-described acceptor material alone or of the above-describedacceptor material and another material in combination. In this case, theacceptor material extracts electrons from the hole-transport layer, sothat holes can be injected into the hole-transport layer. The acceptormaterial transfers the extracted electrons to the anode.

[Electron-Transport Layer]

The electron-transport layer 133 is a layer including a material with ahigh electron-transport property. For the electron-transport layer 133,a metal complex such as Alq₃, tris(4-methyl-8-quinolinolato)aluminum(abbreviation: Almqg),bis(10-hydroxybenzo[h]quinolinato)beryllium(abbreviation:BeBq₂), BAlq,Zn(BOX)₂, or bis[2-(2-hydroxyphenyl)benzothiazolato]zine (abbreviation:Zn(BTZ)₂) can be used. Furthermore, a heteroaromatic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: Bphen,bathocuproine (abbreviation: BCP), or4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs) can alsobe used. Further alternatively, it is possible to use a high molecularcompound such as poly(2,5-pyridinediyl) (abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2-bipyridine-6,6-diyl)](abbreviation: PF-BPy). These materials described here are mainlymaterials each having an electron mobility of greater than or equal to1×10⁻⁶ cm²/Vs. Note that other than the above-described materials, amaterial that has a property of transporting more holes than electronsmay be used. Note that any other material may be used for theelectron-transport layer 133 as long as the material has anelectron-transport property higher than a hole-transport property.

The electron-transport layer 133 is not limited to a single layer, andmay be a stack of two or more layers each containing any of theabove-described materials.

[Electron-Injection Layer]

The electron-injection layer 134 is a layer including a material with ahigh electron-injection property. For the electron-injection layer 134,an alkali metal, an alkaline earth metal, or a compound thereof, such aslithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF₂),or lithium oxide (LiO_(x)), can be used. Alternatively, a rare earthmetal compound like erbium fluoride (ErF₃) can be used. Electride mayalso be used for the electron-injection layer 134. Examples of theelectride include a material in which electrons are added at highconcentration to calcium oxide-aluminum oxide.

Alternatively, the electron-injection layer 134 may be formed using acomposite material in which an organic compound and an electron donor(donor) are mixed. The composite material is superior in anelectron-injection property and an electron-transport property becauseelectrons are generated in the organic compound by the electron donor.The organic compound here is preferably a material excellent intransporting the generated electrons; specifically, for example, thematerials for forming the electron-transport layer 133 (e.g., a metalcomplex or a heteroaromatic compound) can be used. As the electrondonor, a material showing an electron-donating property with respect tothe organic compound may be used. Specifically, an alkali metal, analkaline earth metal, and a rare earth metal are preferable, andlithium, cesium, magnesium, calcium, erbium, ytterbium, and the like aregiven. Furthermore, an alkali metal oxide or an alkaline earth metaloxide is preferable, and for example, lithium oxide, calcium oxide,barium oxide, and the like can be given. Alternatively, Lewis base suchas magnesium oxide can also be used. An organic compound such astetrathiafulvalene (abbreviation: TTF) can also be used.

Note that the above-described light-emitting layer, the hole-transportlayer, the hole-injection layer, the electron-transport layer, and theelectron-injection layer can each be formed by any of the followingmethods: a sputtering method, an evaporation method (including a vacuumevaporation method), a printing method (such as relief printing,intaglio printing, gravure printing, planography printing, and stencilprinting), an ink jet method, a coating method, and the like.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 2

In this embodiment, an emission mechanism of the light-emitting element100 described in Embodiment 1 will be described with reference to FIGS.8A and 8B, FIGS. 9A and 9B, and FIGS. 10A and 10B.

<2-1. Emission Mechanism of Light-Emitting Element>

First, an emission mechanism of the light-emitting element 100 will bedescribed.

In the light-emitting element 100 of one embodiment of the presentinvention, voltage application between a pair of electrodes (the firstelectrode 104 and the second electrode 114) causes electrons and holesto be injected from the cathode and the anode, respectively, into the ELlayer 108 and thus current flows. By recombination of the injectedelectrons and holes, a guest material (a fluorescent material and aphosphorescent material) in the first light-emitting layer 110 and thesecond light-emitting layer 112 is brought into an excited state toprovide light emission.

<2-2. Emission Mechanism 1 of First Light-Emitting Layer>

Next, an emission mechanism of the first light-emitting layer 110 of thelight-emitting element 100 will be described.

FIG. 8A is an example of a schematic cross-sectional view of the firstlight-emitting layer 110. The first light-emitting layer 110 in FIG. 8Aincludes a host material 121 and a guest material 122.

It is preferable that the host material 121 have a difference of 0.2 eVor less between a singlet excitation energy level and a tripletexcitation energy level. It is particularly preferable that the hostmaterial 121 be a material which exhibits thermally activated delayedfluorescence at room temperature. Note that the host material 121 may becomposed of a single material or may include a plurality of materials.The guest material 122 may be a light-emitting organic compound, and thelight-emitting organic compound is preferably a fluorescent material. Anexample in which a fluorescent material is used as the guest material122 will be described below.

Note that light emission from the guest material 122 can be obtainedthrough the following two processes:

(α) direct recombination process; and

(β1) energy transfer process.

<2-3. (α) Direct Recombination Process>

Carriers (electrons and holes) are recombined in the guest material 122,and the guest material 122 is brought into an excited state. In the casewhere the excited state of the guest material 122 is a singlet excitedstate, fluorescence is obtained. In contrast, in the case where theexcited state of the guest material 122 is a triplet excited state,thermal deactivation occurs.

In (α) direct recombination process, high emission efficiency can beobtained when the fluorescence quantum efficiency of the guest material122 is high.

<2-4. (β1) Energy Transfer Process>

Carriers are recombined in the host material 121, and the host material121 is brought into an excited state. In the case where the excitedstate of the host material 121 is a singlet excited state and thesinglet excitation energy level of the host material 121 is higher thanthe singlet excitation energy level of the guest material 122,excitation energy is transferred from the host material 121 to the guestmaterial 122, and thus the guest material 122 is brought into a singletexcited state. Fluorescence is obtained from the guest material 122 inthe singlet excited state. Therefore, the singlet excitation energylevel of the host material 121 is preferably higher than the singletexcitation energy level of the guest material 122.

Note that since direct transition of the guest material 122 from asinglet ground state to a triplet excited state is forbidden, energytransfer from the singlet excited state of the host material 121 to thetriplet excited state of the guest material 122 is unlikely to be a mainenergy transfer process; therefore, a description thereof is omittedhere. In other words, energy transfer from the singlet excited state ofthe host material 121 to the singlet excited state of the guest material122 is important as represented by the following general formula (G1).

^(I) H*+ ¹ G→ ^(I) H+ ^(I) G*  (G1)

Note that in the general formula (G1), ¹H* represents the singletexcited state of the host material 121; ¹G represents the singlet groundstate of the guest material 122; H represents the singlet ground stateof the host material 121; and ¹G* represents the singlet excited stateof the guest material 122.

Next, in order to describe the energy transfer process of the hostmaterial 121 and the guest material 122, a schematic diagramillustrating the correlation of energy levels is shown in FIG. 8B. Thefollowing explains what terms and signs in FIG. 8B represent:

Host (121): the host material 121;

Guest (122): the guest material 122 (fluorescent material);

S_(H): the level of the lowest singlet excitation energy of the hostmaterial 121;

T_(H): the level of the lowest triplet excitation energy of the hostmaterial 121;

S_(G): the level of the lowest singlet excitation energy of the guestmaterial 122 (fluorescent material); and

T_(G): the level of the lowest triplet excitation energy of the guestmaterial 122 (fluorescent material).

Even in the case where the exited state of the host material 121 is thetriplet excited state, when the Su of the host material 121 is higherthan the Sc of the guest material 122, fluorescence is obtained throughthe following two processes.

As for a first process, excitation energy is transferred from the T_(H)to the S_(H) of the host material 121 by reverse intersystem crossing(upconversion) as shown by a route A₁ in FIG. 8B.

As for a subsequent second process, excitation energy is transferredfrom the S_(H) of the host material 121 to the S_(G) of the guestmaterial 122 as shown by a route E₁ in FIG. 8B, whereby the guestmaterial 122 is brought into the singlet excited state. Fluorescence isobtained from the guest material 122 in the singlet excited state.

The above-described first and second processes are represented by thefollowing general formula (G2).

¹ H+ ¹ G→(reverse intersystem crossing)→¹ H*+ ¹ G→ ¹ H+ ¹ G*  (G2)

Note that in the general formula (G2), ³H* represents the tripletexcited state of the host material 121; G represents the singlet groundstate of the guest material 122; ¹H* represents the singlet excitedstate of the host material 121; ¹H represents the singlet ground stateof the host material 121; and ¹G* represents the singlet excited stateof the guest material 122.

As represented by the general formula (G2), the singlet excited state(¹H*) of the host material 121 is generated from the triplet excitedstate (³H*) of the host material 121 by reverse intersystem crossing,and then energy is transferred to the guest material 122 in the singletexcited state(¹G*).

When all the energy transfer processes described above in (β1) energytransfer process occur efficiently, both the triplet excitation energyand the singlet excitation energy of the host material 121 areefficiently converted into the singlet excited state (¹G*) of the guestmaterial 122. Thus, high-efficiency light emission is possible.

However, when the host material 121 is deactivated by emittingexcitation energy as light or heat before the excitation energy istransferred from the singlet excited state and the triplet excited stateof the host material 121 to the singlet excited state of the guestmaterial 122, the emission efficiency is decreased. For example, in thecase where the level of the lowest triplet excitation energy of the hostmaterial 121 is lower than the level of the lowest triplet excitationenergy of the guest material 122 as indicated by broken line B₁ in FIG.8B, thermal deactivation occur through an energy transfer process shownby a route E₃ in FIG. 8B. In that case, since there is a large energydifference between T_(H) and S_(H), reverse intersystem crossing shownby the route A₁ in FIG. 8B and the subsequent energy transfer processshown by the route E₁ are unlikely to occur, which reduces thegeneration efficiency of the singlet excited state of the guest material122. Thus, it is preferable that the T_(H) of the host material 121 behigher than the T_(G) of the guest material 122. That is, in the casewhere the host material 121 is a material which exhibits thermallyactivated delayed fluorescence, it is preferable that the thermallyactivated delayed fluorescence emission energy of the host material 121be higher than the phosphorescence emission energy of the guest material122.

At this time, in the case where excitation energy is transferred fromthe T_(H) of the host material 121 to the T_(G) of the guest material122 as shown by a route E₂ in FIG. 8B, the excitation energy is alsothermally deactivated. Therefore, it is preferable that the energytransfer process shown by the route E₂ in FIG. 8B be less likely tooccur because the generation efficiency of the triplet excited state ofthe guest material 122 can be decreased and the occurrence of thermaldeactivation can be reduced. To achieve this, it is preferable that theconcentration of the guest material 122 with respect to the hostmaterial 121 be low. Specifically, the concentration of the guestmaterial 122 with respect to the host material 121 is preferably morethan 0 wt % and less than or equal to 5 wt % and further preferably morethan 0 wt % and less than or equal to 1 wt.

Note that when the direct recombination process in the guest material122 is dominant, a large number of triplet excited states of the guestmaterial 122 are generated in the light-emitting layer, resulting in adecreased emission efficiency due to thermal deactivation. That is, itis preferable that the probability of (β1) energy transfer process behigher than that of (α) direct recombination process because theoccurrence of thermal deactivation when the excited state of the guestmaterial 122 is a triplet excited state can be reduced. To achieve this,it is again preferable that the concentration of the guest material 122with respect to the host material 121 be low. Specifically, theconcentration of the guest material 122 with respect to the hostmaterial 121 is preferably more than 0 wt % and less than or equal to 5wt % and further preferably more than 0 wt % and less than or equal to 1wt %.

Next, factors controlling the above-described processes ofintermolecular energy transfer between the host material 121 and theguest material 122 will be described. As mechanisms of theintermolecular energy transfer, two mechanisms, i.e., Förster mechanism(dipole-dipole interaction) and Dexter mechanism (electron exchangeinteraction), have been proposed.

<2-5. Förster Mechanism>

In Förster mechanism, energy transfer does not require direct contactbetween molecules and energy is transferred through a resonantphenomenon of dipolar oscillation between the host material 121 and theguest material 122. By the resonant phenomenon of dipolar oscillation,the host material 121 provides energy to the guest material 122, andthus, the host material 121 in an excited state is put in a ground stateand the guest material 122 in a ground state is put in an excited state.Note that the rate constant k_(h*→g) of Förster mechanism is expressedby Formula (1).

$\begin{matrix}{k_{h^{*}->g} = {\frac{9000K^{2}{\varphi ln10}}{128\pi^{5}n^{4}{N\tau R}^{6}}{\int{\frac{{{f^{\prime}}_{h}(v)}{ɛ_{g}(v)}}{v^{4}}{dv}}}}} & (1)\end{matrix}$

In Formula (1), 1 denotes a frequency, f′_(h)(v) denotes a normalizedemission spectrum of the host material 121 (a fluorescent spectrum inenergy transfer from a singlet excited state, and a phosphorescentspectrum in energy transfer from a triplet excited state), ε_(g)(v)denotes a molar absorption coefficient of the guest material 122, Ndenotes Avogadro's number, n denotes a refractive index of a medium, Rdenotes an intermolecular distance between the host material 121 and theguest material 122, t denotes a measured lifetime of an excited state(fluorescence lifetime or phosphorescence lifetime), ϕ denotes aluminescence quantum yield (a fluorescence quantum yield in energytransfer from a singlet excited state, and a phosphorescence quantumyield in energy transfer from a triplet excited state), and K² denotes acoefficient (0 to 4) of orientation of a transition dipole momentbetween the host material 121 and the guest material 122. Note that K²=⅔in random orientation.

<2-6. Dexter Mechanism>

In Dexter mechanism, the host material 121 and the guest material 122are close to a contact effective range where their orbitals overlap, andthe host material 121 in an excited state and the guest material 122 ina ground state exchange their electrons, which leads to energy transferNote that the rate constant k_(h*→g) of Dexter mechanism is expressed byFormula (2).

$\begin{matrix}{k_{h^{*}->g} = {( \frac{2\pi}{h} )K^{2}{\exp ( {- \frac{2R}{L}} )}{\int{{{f^{\prime}}_{h}(v)}{{ɛ^{\prime}}_{g}(v)}{dv}}}}} & (2)\end{matrix}$

In Formula (2), h denotes a Planck constant, K denotes a constant havingan energy dimension, v denotes a frequency, f′_(h)(v) denotes anormalized emission spectrum of the host material 121 (a fluorescentspectrum in energy transfer from a singlet excited state, and aphosphorescent spectrum in energy transfer from a triplet excitedstate), ε′_(g)(v) denotes a normalized absorption spectrum of the guestmaterial 122, L denotes an effective molecular radius, and R denotes anintermolecular distance between the host material 121 and the guestmaterial 122.

Here, the efficiency of energy transfer from the host material 121 tothe guest material 122 (energy transfer efficiency ϕ_(ET)) is thought tobe expressed by Formula (3). In the formula, k_(r) denotes a rateconstant of a light-emission process (fluorescence in energy transferfrom a singlet excited state, and phosphorescence in energy transferfrom a triplet excited state) of the host material 121, k_(n) denotes arate constant of a non-light-emission process (thermal deactivation orintersystem crossing) of the host material 121, and τ denotes a measuredlifetime of an excited state of the host material 121.

$\begin{matrix}{\varphi_{ET} = {\frac{k_{h^{*}->g}}{k_{r} + k_{n} + k_{h^{*}->g}} = \frac{k_{h^{*}->g}}{( \frac{1}{\tau} ) + k_{h^{*}->g}}}} & (3)\end{matrix}$

According to Formula (3), it is found that the energy transferefficiency ϕ_(ET) can be increased by increasing the rate constantk_(h*→g) of energy transfer so that another competing rate constantk_(r)+k_(n) (=1/τ) becomes relatively small.

<2-7. Concept for Promoting Energy Transfer>

In both the energy transfer processes of the general formulae (G1) and(G2), since energy is transferred from the singlet excited state (¹H*)of the host material 121 to the singlet excited state (¹G*) of the guestmaterial 122, energy transfers by both Forster mechanism (Formula (1))and Dexter mechanism (Formula (2)) are possible.

First, an energy transfer by Forster mechanism is considered. When c iseliminated from Formula (1) and Formula (3), it can be said that theenergy transfer efficiency ϕ_(ET) is higher when the quantum yield ϕ(here, a fluorescence quantum efficiency because energy transfer from asinglet excited state is discussed) is higher. However, in practice, amore important factor is that the emission spectrum of the host material121 (here, a fluorescent spectrum because energy transfer from a singletexcited state is discussed) largely overlaps with the absorptionspectrum of the guest material 122 (absorption corresponding to thetransition from the singlet ground state to the singlet excited state).Note that it is preferable that the molar absorption coefficient of theguest material 122 be also high. This means that the emission spectrumof the host material 121 overlaps with the absorption band of the guestmaterial 122 which is on the longest wavelength side.

Next, an energy transfer by Dexter mechanism is considered. According toFormula (2), in order to increase the rate constant k_(h*→g), it ispreferable that an emission spectrum of the host material 121 (here, afluorescent spectrum because energy transfer from a singlet excitedstate is discussed) largely overlap with an absorption spectrum of theguest material 122 (absorption corresponding to transition from asinglet ground state to a singlet excited state).

The above description suggests that the energy transfer efficiency canbe optimized by making the emission spectrum of the host material 121overlap with the absorption band of the guest material 122 which is onthe longest wavelength side.

It is preferable that the host material 121 have a difference of 0.2 eVor less between a singlet excitation energy level and a tripletexcitation energy level. This enables transition (reverse intersystemcrossing) of the host material 121 from the triplet excited state to thesinglet excited state to be likely to occur. Therefore, the generationefficiency of the singlet excited state of the host material 121 can beincreased. Furthermore, it is preferable that the emission spectrum ofthe host material 121 (here, the emission spectrum of a material havinga function of exhibiting thermally activated delayed fluorescence)overlap with the absorption band of the guest material 122 having afunction as an energy acceptor, which is on the longest wavelength side.This facilitates energy transfer from the singlet excited state of thehost material 121 to the singlet excited state of the guest material122. Thus, the generation efficiency of the singlet excited state of theguest material 122 can be increased.

Since the triplet excitation energy level of the host material 121 ishigher than the triplet excitation energy level of the guest material122, transition of the host material 121 from the triplet excited stateto the singlet excited state and energy transfer from the singletexcited state of the host material 121 to the singlet excited state ofthe guest material 122 are likely to occur. For this reason, thermaldeactivation is less likely to occur; thus, the emission efficiency canbe increased. In the case where the host material 121 is a materialwhich exhibits thermally activated delayed fluorescence at roomtemperature, since the thermally activated delayed fluorescence emissionenergy is higher than the phosphorescence emission energy of the guestmaterial 122, transition of the host material 121 from the tripletexcited state to the singlet excited state and energy transfer from thesinglet excited state of the host material 121 to the singlet excitedstate of the guest material 122 occur efficiently. For this reason,thermal deactivation is less likely to occur; thus, the emissionefficiency can be increased.

<2-8. Emission Mechanism 2 of First Light-Emitting Layer>

Next, an emission mechanism different from <2,2. Emission mechanism 1 offirst light-emitting layer> will be described below with reference toFIGS. 9A and 9B.

FIG. 9A is an example of a schematic cross-sectional view of the firstlight-emitting layer 110. The first light-emitting layer 110 in FIG. 9Aincludes the host material 121 and the guest material 122. The hostmaterial 121 includes a first organic compound 121 . . . 1 and a secondorganic compound 121_2.

It is preferable that a combination of the first organic compound 121_1and the second organic compound 121_2 form an exciplex (also referred toas an excited complex). An exciplex tends to have a very smalldifference between the singlet excitation energy level and the tripletexcitation energy level, and thus transition (reverse intersystemcrossing) from the triplet excited state to the singlet excited state islikely to occur. One of the first organic compound 1211 and the secondorganic compound 121_2 serves as a host material for the firstlight-emitting layer 110, and the other of the first organic compound121 . . . 1 and the second organic compound 1212 serves as an assistmaterial for the first light-emitting layer 110. Note that the firstorganic compound 121 . . . 1 serves as the host material and the secondorganic compound 121_2 serves as the assist material in the followingdescription.

Note that also in the case of using a host material which allows acombination of the first organic compound 121 . . . 1 and the secondorganic compound 121 . . . 2 to form an exciplex, light emission fromthe guest material 122 can be obtained through the following twoprocesses:

(α) direct recombination process; and

(β2) energy transfer process.

Note that (α) direct recombination process is not described here becauseit is similar to the process described above in the subsection 2-3.

<2-9. (β2) Energy Transfer Process>

Although there is no limitation on the combination of the first organiccompound 121_1 and the second organic compound 121_2 in the firstlight-emitting layer 110 as long as an exciplex can be formed, it ispreferred that one organic compound be a material having ahole-transport property and the other organic compound be a materialhaving an electron-transport property. In that case, a donor-acceptorexcited state is formed easily, which allows an exciplex to be formedefficiently. In the case where the combination of the first organiccompound 121_I and the second organic compound 121_2 is a combination ofthe material having a hole-transport property and the material having anelectron-transport property, the carrier balance can be easilycontrolled depending on the mixture ratio. Specifically, the weightratio of the material having a hole-transport property to the materialhaving an electron-transport property is preferably within the range of1:9 to 9:1. Since the carrier balance can be easily controlled in thestructure, a recombination region can also be easily adjusted.

It is preferable that the exciplex formed by the first organic compound121_I and the second organic compound 121_2 have a difference of 0.2 eVor less between the singlet excitation energy level and the tripletexcitation energy level. This enables transition of the exciplex fromthe triplet excitation energy level to the singlet excitation energylevel to be likely to occur. Therefore, the generation efficiency of thesinglet excited state of the exciplex, i.e., the host material 121 canbe increased. Furthermore, it is preferable that the emission spectrumof the host material 121 (here, the emission spectrum of the exciplexformed by the first organic compound 121_1 and the second organiccompound 121_2) overlap with the absorption band of the guest material122 which is on the longest wavelength side. This facilitates energytransfer from the singlet excited state of the host material 121 to thesinglet excited state of the guest material 122. Therefore, thegeneration efficiency of the singlet excited state of the guest material122 can be increased; thus, emission efficiency can be increased.

Here, in order to describe the energy transfer process of the exciplex,a schematic diagram illustrating the correlation of energy levels isshown in FIG. 9B. The following explains what terms and signs in FIG. 9Brepresent:

Host (121): the host material 121;

Guest (122): the guest material 122 (fluorescent material);

S_(H): the level of the lowest singlet excitation energy of the hostmaterial 121;

T_(H): the level of the lowest triplet excitation energy of the hostmaterial 121;

S_(E): the level of the lowest singlet excitation energy of theexciplex;

T_(E): the level of the lowest triplet excitation energy of theexciplex;

S_(G): the level of the lowest singlet excitation energy of the guestmaterial 122 (fluorescent material); and

T_(G): the level of the lowest triplet excitation energy of the guestmaterial 122 (fluorescent material).

When carriers are transported to the first light-emitting layer 110, oneof the first organic compound 121_1 and the second organic compound121_2 receives holes and the other receives electrons, and a cation andan anon come close to each other, whereby the exciplex is formed atonce. Alternatively, when one compound is brought into an excited state,the one immediately interacts with the other compound to form theexciplex. Therefore, most excitons in the first light-emitting layer 110exist as the exciplexes. The band gap of the exciplex is narrower thanthat of each of the first organic compound 121_1 and the second organiccompound 121_2; therefore, the driving voltage can be lowered when theexciplex is formed by recombination of a hole and an electron.

As shown in FIG. 9B, the first organic compound 1211 and the secondorganic compound 121_2 included in the host material 121 form theexciplex. Since a donor-acceptor excited state can be formed at thistime, the S_(E) and the T_(E) of the exciplex are close to each other.

In the case where the excited state of the exciplex is a single excitedstate, excitation energy is transferred from the S_(E) of the exciplexto the S_(G) of the guest material 122 as shown by a route E₄ in FIG.9B, whereby the guest material 122 is brought into the singlet excitedstate. Fluorescence is obtained from the guest material 122 in thesinglet excited state. In other words, energy transfer occurs from thesinglet excited state of the exciplex to the singlet excited state ofthe guest material 122 as represented by the following general formula(G3).

¹[H−A]*+¹ G→H+ ¹ A+ ¹ G*  (G3)

Note that in the general formula (G3), ¹[H−A]* represents the singletexcited state of the exciplex formed by the first organic compound 121 .. . 1 and the second organic compound 121_2; G represents the singletground state of the guest material 122; ¹H represents the singlet groundstate of the first organic compound 121_1; ¹A represents the singletground state of the second organic compound 121_2; and ¹G* representsthe singlet excited state of the guest material 122.

Even in the case where the exited state of the exciplex is the tripletexcited state, when the S_(E) of the exciplex is higher than the S_(G)of the guest material 122, fluorescence is obtained through thefollowing two processes.

As for a first process, excitation energy is transferred from the T_(E)to the S_(E) of the exciplex by reverse intersystem crossing(upconversion) as shown by a route A₂ in FIG. 9B.

As for a subsequent second process, excitation energy is transferredfrom the S_(E) of the exciplex to the S_(G) of the guest material 122 asshown by a route E₄ in FIG. 9B, whereby the guest material 122 isbrought into the singlet excited state. Fluorescence is obtained fromthe guest material 122 in the singlet excited state.

The above-described processes through the route A₂ and the route E₄ maybe referred to as exciplex-singlet energy transfer (ExSBT) orexciplex-enhanced fluorescence (ExEF) in this specification and thelike.

The above-described first and second processes are represented by thefollowing general formula (G4).

³[H−A]*+¹ G→(reverse intersystem crossing)→¹[H−A]*+¹ G→ ¹ H+ ¹ A+ ¹G*  (G4)

Note that in the general formula (G4), ³[H−A]* represents the tripletexcited state of the exciplex formed by the first organic compound 1211and the second organic compound 121_2; 1G represents the singlet groundstate of the guest material 122; ¹[H−A]* represents the singlet excitedstate of the exciplex formed by the first organic compound 121_1 and thesecond organic compound 121_2; ¹H represents the singlet ground state ofthe first organic compound 121_1; ¹A represents the singlet ground stateof the second organic compound 121_2; and ¹G* represents the singletexcited state of the guest material 122.

As represented by the general formula (G4), the singlet excited state(¹[H−A]*) of the exciplex is generated from the triplet excited state(³[H−A]*) of the exciplex by reverse intersystem crossing, and thenenergy is transferred to the singlet excited state (¹G*) of the guestmaterial 122.

When the host material 121 has the above structure, (β2) energy transferprocess occurs efficiently, and both the singlet excitation energy andthe triplet excitation energy of the exciplex are efficiently convertedinto the singlet excited state of the guest material 122. Thus, lightemission can be efficiently obtained from the guest material 122(fluorescent material) of the first light-emitting layer 110.

However, when the exciplex is deactivated by emitting the excitationenergy as light or heat before excitation energy is transferred from theexciplex to the guest material 122, the emission efficiency may bedecreased. For example, in the case where excitation energy istransferred from the T_(E) of the exciplex to the T_(G) of the guestmaterial 122 as shown by a route E₅ in FIG. 9B, the excitation energy isthermally deactivated. Therefore, the concentration of the guestmaterial 122 to the host material 121 is preferably more than 0 wt % andless than or equal to 5 wt % and further preferably more than 0 wt % andless than or equal to 1 wt %.

In the case where the T_(H) of the host material 121, i.e., the tripletexcitation energy level of the first organic compound 121_1 or thesecond organic compound 1212 is lower than the T_(B) of the exciplex asindicated by broken line B₂ in FIG. 9B, thermal deactivation occursthrough an energy transfer process shown by a route E₆ in FIG. 9B. Thus,it is preferable that the triplet excitation energy level of each of thefirst organic compound 121 . . . 1 and the second organic compound 121_2be higher than the Ta of the exciplex. Since the S_(E) and T_(E) of theexciplex are close to each other, in the case where the T_(E) is lowerthan the Ta of the guest material 122, the energy level of S_(E) issignificantly lowered to the vicinity of T_(G) or lower than T_(G). As aresult, energy transfer from the S_(E) to the S_(G) of the guestmaterial 122 (route E) is unlikely to occur, and fluorescence is noteasily obtained from the guest material 122. Thus, it is preferable thatthe T_(E) of the exciplex be higher than the T_(G) of the guest material122. Accordingly, in the case where the exciplex exhibits thermallyactivated delayed fluorescence at room temperature, it is preferablethat the phosphorescence emission energy of each of the first organiccompound 121_1 and the second organic compound 121_2 be higher than thethermally activated delayed fluorescence emission energy of theexciplex. It is also preferable that the thermally activated delayedfluorescence emission energy of the exciplex be higher than thephosphorescence emission energy of the guest material 122.

Note that either <2-2. Emission mechanism 1 of first light-emittinglayer> or <2-8. Emission mechanism 2 of first light-emitting layer> ispreferably used for the first light-emitting layer 110 because theemission efficiency of the first light-emitting layer 110 can beincreased.

<2-10. Emission Mechanism of Second Light-Emitting Layer>

Next, an emission mechanism of the second light-emitting layer 112 ofthe light-emitting element 100 will be described.

FIG. 10A is an example of a schematic cross-sectional view of the secondlight-emitting layer 112. The second light-emitting layer 112 in FIG.10A includes a host material 221 and a guest material 222. The hostmaterial 221 includes a third organic compound 221_1 and a fourthorganic compound 2212.

The third organic compound 221_1 and the fourth organic compound 221_2of the second light-emitting layer 112 form an exciplex. The thirdorganic compound 2211 serves as a host material and the fourth organiccompound 2212 serves as an assist material in the description hem.

Although there is no limitation on the combination of the third organiccompound 221_1 and the fourth organic compound 221_2 in the secondlight-emitting layer 112 as long as an exciplex can be formed, it ispreferred that one organic compound be a material having ahole-transport property and the other organic compound be a materialhaving an electron-transport property. Note that the combination of thethird organic compound 221_1 and the fourth organic compound 221_2 mayhave a structure similar to the combination of the first organiccompound 121_1 and the second organic compound 121_2 which form anexciplex in the first light-emitting layer 110.

FIG. 10B illustrates the correlation of energy levels of the thirdorganic compound 221_1, the fourth organic compound 221_2, and the guestmaterial 222 in the second light-emitting layer 112. The followingexplains what terms and signs in FIG. 10B represent:

Host (221_1): the host material (third organic compound 221_1);

Assist (221_2): the assist material (fourth organic compound 221_2);

Guest (222): the guest material 222 (phosphorescent material);

S_(PH): the level of the lowest singlet excited state of the hostmaterial (third organic compound 221_1);

T_(PH): the level of the lowest triplet excited state of the hostmaterial (third organic compound 221_1);

T_(PG): the level of the lowest triplet excited state of the guestmaterial 222 (the phosphorescent material);

S_(PE): the level of the lowest singlet excited state of the exciplex;and

T_(PE): the level of the lowest triplet excited state of the exciplex.

As shown by a route E₇ in FIG. 10B, the level (S_(PE)) of the lowestsinglet excited state of the exciplex, which is formed by the thirdorganic compound 221_1 and the fourth organic compound 221_2, and thelevel (T_(PE)) of the lowest triplet excited state of the exciplex areclose to each other.

Both energies of S_(PE) and T_(PE) of the exciplex are then transferredto the level (T_(PG)) of the lowest triplet excited state of the guestmaterial 222 (the phosphorescent material) as shown by a route E₈ inFIG. 10B; thus, light emission is obtained.

The above-escribed processes through the route E₇ and the route E₈ maybe referred to as exciplex-triplet energy transfer (ExTET) in thisspecification and the like.

When one of the third organic compound 221_1 and the fourth organiccompound 221_2 receiving holes and the other receiving electrons comeclose to each other, the exciplex is formed at once. Alternatively, whenone compound is brought into an excited state, the one immediately takesin the other compound to form the exciplex. Therefore, most excitons inthe second light-emitting layer 112 exist as the exciplexes. The bandgap of the exciplex is narrower than that of each of the third organiccompound 221_1 and the fourth organic compound 221_2; therefore, thedriving voltage can be lowered when the exciplex is formed byrecombination of a hole and an electron.

When the second light-emitting layer 112 has the above structure, lightemission from the guest material 222 (the phosphorescent material) ofthe second light-emitting layer 112 can be efficiently obtained.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

EMBODIMENT 3

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIG. 11, FIG. 12,and FIG. 13. Note that FIG. 11 is a schematic cross-sectional view of alight-emitting element 150 of one embodiment of the present invention,and FIG. 12 and FIG. 13 are schematic cross-sectional views of alight-emitting element 150A of one embodiment of the present invention.

<3-1. Structure 2 of Light-Emitting Element>

The light-emitting element 150 illustrated in FIG. 11 includes aplurality of EL layers (a first EL layer 141 and a second EL layer 142)between the first electrode 104 and the second electrode 114. One orboth of the first EL layer 141 and the second EL layer 142 have the samestructure as the EL layer 108 illustrated in FIGS. 1A and 1B. That is,the light-emitting element 100 in FIGS. 1A and 1B includes one EL layerwhile the light-emitting element 150 includes the plurality of ELlayers. Note that in this specification and the like, an EL layerincludes at least a light-emitting material.

In the light-emitting element 150 in FIG. 11, the first EL layer 141 andthe second EL layer 142 are stacked, and a charge generation layer 143is provided between the first EL layer 141 and the second EL layer 142.Note that the first EL layer 141 and the second EL layer 142 may havethe same structure or different structures.

The charge generation layer 143 includes a composite material of anorganic compound and a metal oxide. For the composite material, thecomposite material that can be used for the hole-injection layer 131described above may be used. As the organic compound, a variety ofcompounds such as an aromatic amine compound, a carbazole compound, anaromatic hydrocarbon, and a high molecular compound (such as anoligomer, a dendrimer, or a polymer) can be used. An organic compoundhaving a hole mobility of 1×10⁻⁶ cm²Ns or higher is preferably used.Note that any other substance may be used as long as the substance has ahole-transport property higher than an electron-transport property. Thecomposite material of an organic compound and a metal oxide is superiorin carrier-injecting property and carrier-transporting property;therefore, low-voltage driving or low-current driving can be achieved.Note that when a surface of the EL layer 141 or the EL layer 142 on theanode side is in contact with the charge generation layer 143, thecharge generation layer 143 can also serve as a hole-transport layer ofthe EL layer 141 or the EL layer 142; thus, a hole-transport layer doesnot need to be formed in the EL layer 141 or the EL layer 142.

The charge generation layer 143 may have a stacked-layer structure of alayer containing the composite material of an organic compound and ametal oxide and a layer containing another material. For example, thecharge generation layer 143 may be formed using a combination of a layercontaining the composite material of an organic compound and a metaloxide with a layer containing one compound selected from amongelectron-donating substances and a compound having a highelectron-transporting property. Further, the charge generation layer 143may be formed using a combination of a layer containing the compositematerial of an organic compound and a metal oxide with a transparentconductive film.

In any case, as the charge-generation layer 143, which is providedbetween the first EL layer 141 and the second EL layer 142, acceptableis a layer which injects electrons into the EL layer on one side andinjects holes into the EL layer on the other side when voltage isapplied to the first electrode 104 and the second electrode 114. Forexample, in FIG. 11, when a voltage is applied such that a potential ofthe first electrode 104 is higher than a potential of the secondelectrode 114, any structure may be used for the charge generation layer143, as long as the charge generation layer 143 injects electrons andholes into the first EL layer 141 and the second EL layer 142,respectively.

In FIG. 11, the light-emitting element having two EL layers isdescribed; however, one embodiment of the present invention can besimilarly applied to a light-emitting element in which three or more ELlayers are stacked. With a plurality of EL layers partitioned by thecharge generation layer 143 between a pair of electrodes as in thelight-emitting element 150, light with high luminance can be obtainedwhile current density is kept low; thus, a light-emitting element with along lifetime can be obtained. A light-emitting device that can bedriven at a low voltage and has low power consumption can be achieved.

When the EL layer 108 described in Embodiment 1 is included in at leastone of the plurality of EL layers, a light-emitting element with highefficiency and a long lifetime can be provided.

<3-2. Structure 3 of Light-Emitting Element>

Next, specific examples of the light-emitting element 150 in FIG. 11will be described with reference to FIG. 12 and FIG. 13.

The light-emitting element 150A illustrated in FIG. 12 includes thefirst EL layer 141 and the second EL layer 142 between the firstelectrode 104 and the second electrode 114. The first EL layer 141 inFIG. 12 has the same structure as the EL layer 108 in FIGS. 1A and 1B.The second EL layer 142 in FIG. 12 includes a hole-injection layer 415,a hole-transport layer 416, a third light-emitting layer 444, anelectron-transport layer 417, and an electron-injection layer 418.

The hole-injection layer 415, the hole-transport layer 416, theelectron-transport layer 417, and the electron-injection layer 418 havethe same structures as the hole-injection layer 131, the hole-transportlayer 132, the electron-transport layer 133, and the electron-injectionlayer 134 described above, respectively.

The third light-emitting layer 444 includes a host material 431 and aguest material 432. The host material 431 includes a first organiccompound 4311 and a second organic compound 431_2. For example, thefirst organic compound 431_1 can be used as a host material and thesecond organic compound 431_2 can be used as an assist material.Although, in this embodiment, two kinds of organic compounds (the firstorganic compound 431_1 and the second organic compound 431_2) are usedfor the host material 431, for example, one kind or three or more kindsof materials may be used without limitation to such a structure.

A phosphorescent material is preferable as the guest material 432.Moreover, the guest material 432 preferably has an emission spectrumhaving a peak different from those of the first light-emitting layer 110and the second light-emitting layer 112. When the first light-emittinglayer 110, the second light-emitting layer 112, and the thirdlight-emitting layer 444 emit light of complementary colors, white lightemission can be obtained, for example. For example, in the case wherethe first light-emitting layer 110 and the second light-emitting layer112 each have a peak of an emission spectrum in a blue wavelength range,it is preferable to use a material whose emission spectrum has a peak ina yellow wavelength range as the guest material 432 of the thirdlight-emitting layer 444.

Alternatively, the light-emitting element 150A in FIG. 12 may have astructure in FIG. 13.

The light-emitting element 150A illustrated in FIG. 13 is different fromthe light-emitting element 150A in FIG. 12 in the structure of thesecond EL layer 142. The light-emitting element 150A in FIG. 13 furtherincludes a fourth light-emitting layer 445 over the third light-emittinglayer 444.

The fourth light-emitting layer 445 of the second EL layer 142 in thelight-emitting element 150A in FIG. 13 includes a host material 441 anda guest material 442. The host material 441 includes a first organiccompound 441_1 and a second organic compound 4412. For example, thefirst organic compound 4411 can be used as a host material and thesecond organic compound 441_2 can be used as an assist material.

A phosphorescent material is preferable as the guest material 442.Moreover, the guest material 442 preferably has an emission spectrumhaving a peak different from those of the first light-emitting layer110, the second light-emitting layer 112, and the third light-emittinglayer 444. For example, the first light-emitting layer 110 and thesecond light-emitting layer 112 each have a peak of an emission spectrumin a blue wavelength range, the third light-emitting layer 444 has apeak of an emission spectrum in a green wavelength range, and the fourthlight-emitting layer 445 has a peak of an emission spectrum in a redwavelength range.

Note that the emission mechanisms and materials of the thirdlight-emitting layer 444 and the fourth light-emitting layer 445 may bethe same as those of the second light-emitting layer 112 describedabove.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 4

In this embodiment, a display device including a light-emitting deviceof one embodiment of the present invention will be described withreference to FIGS. 14A and 14B

<4. Display Device>

FIG. 14A is a block diagram illustrating the display device of oneembodiment of the present invention, and FIG. 14B is a circuit diagramillustrating a pixel circuit of the display device of one embodiment ofthe present invention.

The display device illustrated in FIG. 14A includes a region includingpixels of display elements (the region is hereinafter referred to as apixel portion 802), a circuit portion provided outside the pixel portion802 and including circuits for driving the pixels (the portion ishereinafter referred to as a driver circuit portion 804), circuitshaving a function of protecting elements (the circuits are hereinafterreferred to as protection circuits 806), and a terminal portion 807.Note that the protection circuits 806 are not necessarily provided.

Part or the whole of the driver circuit portion 804 is preferably formedover a substrate over which the pixel portion 802 is formed, in whichcase the number of components and the number of terminals can bereduced. When part or the whole of the driver circuit portion 804 is notformed over the substrate over which the pixel portion 802 is formed,the part or the whole of the driver circuit portion 804 can be mountedby COG or tape automated bonding (TAB).

The pixel portion 802 includes a plurality of circuits for drivingdisplay elements arranged in X rows (X is a natural number of 2 or more)and Y columns (Y is a natural number of 2 or more) (such circuits arehereinafter referred to as pixel circuits 801). The driver circuitportion 804 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (the circuit is hereinafterreferred to as a gate driver 804 a) and a circuit for supplying a signal(data signal) to drive a display element in a pixel (the circuit ishereinafter referred to as a source driver 804 b).

The gate driver 804 a includes a shift register or the like. Through theterminal portion 807, the gate driver 804 a receives a signal fordriving the shift register and outputs a signal. For example, the gatedriver 804 a receives a start pulse signal, a clock signal, or the likeand outputs a pulse signal. The gate driver 804 a has a function ofcontrolling the potentials of wirings supplied with scan signals (suchwirings are hereinafter referred to as scan lines GL_1 to GL_X). Notethat a plurality of gate drivers 804 a may be provided to control thescan lines GL_1 to GL_X separately. Alternatively, the gate driver 804 ahas a function of supplying an initialization signal. Without beinglimited thereto, the gate driver 804 a can supply another signal.

The source driver 804 b includes a shift register or the like. Thesource driver 804 b receives a signal (image signal) from which a datasignal is derived, as well as a signal for driving the shift register,through the terminal portion 807. The source driver 804 b has a functionof generating a data signal to be written to the pixel circuit 801 whichis based on the image signal. In addition, the source driver 804 b has afunction of controlling output of a data signal in response to a pulsesignal produced by input of a start pulse signal, a clock signal, or thelike. Furthermore, the source driver 804 b has a function of controllingthe potentials of wirings supplied with data signals (such wirings arehereinafter referred to as data lines DL_1 to DL_Y). Alternatively, thesource driver 804 b has a function of supplying an initializationsignal. Without being limited thereto, the source driver 804 b cansupply another signal.

The source driver 804 b includes a plurality of analog switches or thelike, for example. The source driver 804 b can output, as the datasignals, signals obtained by time-dividing the image signal bysequentially turning on the plurality of analog switches. The sourcedriver 804 b may include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuits 801 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of data lines DLsupplied with data signals, respectively. Writing and holding of thedata signal to and in each of the plurality of pixel circuits 801 arecontrolled by the gate driver 804 a. For example, to the pixel circuit801 in the m-th row and the n-th column (m is a natural number of lessthan or equal to X, and n is a natural number of less than or equal toY), a pulse signal is input from the gate driver 804 a through the scanline GL_m, and a data signal is input from the source driver 804 bthrough the data line DL_n in accordance with the potential of the scanline GL_m.

The protection circuit 806 illustrated in FIG. 14A is connected to, forexample, the scan line GL between the gate driver 804 a and the pixelcircuit 801. Alternatively, the protection circuit 806 is connected tothe data line DL between the source driver 804 b and the pixel circuit801. Alternatively, the protection circuit 806 can be connected to awiring between the gate driver 804 a and the terminal portion 807.Alternatively, the protection circuit 806 can be connected to a wiringbetween the source driver 804 b and the terminal portion 807. Note thatthe terminal portion 807 means a portion having terminals for inputtingpower, control signals, and image signals to the display device fromexternal circuits.

The protection circuit 806 is a circuit that electrically connects awiring connected to the protection circuit to another wiring when apotential out of a certain range is applied to the wiring connected tothe protection circuit.

As illustrated in FIG. 14A, the protection circuits 806 are provided forthe pixel portion 802 and the driver circuit portion 804, so that theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like can be improved. Note that theconfiguration of the protection circuits 806 is not limited to that, andfor example, a configuration in which the protection circuits 806 areconnected to the gate driver 804 a or a configuration in which theprotection circuits 806 are connected to the source driver 804 b may beemployed. Alternatively, the protection circuits 806 may be configuredto be connected to the terminal portion 807.

Although, in FIG. 14A, the driver circuit portion 804 includes the gatedriver 804 a and the source driver 804 b, for example, the structure isnot limited thereto. For example, only the gate driver 804 a may beformed and a separately prepared substrate where a source driver isformed (e.g., a driver circuit substrate formed with a single crystalsemiconductor film or a polycrystalline semiconductor film) may bemounted.

Each of the plurality of pixel circuits 801 in FIG. 14A can have astructure illustrated in FIG. 14B, for example.

The pixel circuit 801 illustrated in FIG. 14B includes transistors 852and 854, a capacitor 862, and a light-emitting element 872.

One of a source electrode and a drain electrode of the transistor 852 iselectrically connected to a wiring to which a data signal is supplied(hereinafter referred to as a signal line DL_n). A gate electrode of thetransistor 852 is electrically connected to a wiring to which a gatesignal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 852 has a function of controlling whether to write a datasignal by being turned on or off.

One of a pair of electrodes of the capacitor 862 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other iselectrically connected to the other of a source electrode and a drainelectrode of the transistor 852.

The capacitor 862 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 854 iselectrically connected to the potential supply line VL_a. Furthermore, agate electrode of the transistor 854 is electrically connected to theother of the source electrode and the drain electrode of the transistor852.

One of an anode and a cathode of the light-emitting element 872 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 854.

As the light-emitting element 872, the light-emitting element 100described in Embodiment 1 can be used.

Note that a high power supply potential VDD is supplied to one of thepotential supply line VL_a and the potential supply line VL_b, and a lowpower supply potential VSS is supplied to the other.

In the display device including the pixel circuits 801 in FIG. 14B, thepixel circuits 801 are sequentially selected row by row by the gatedriver 804 a in FIG. 14A, for example, whereby the transistors 852 areturned on and a data signal is written.

When the transistors 852 are turned off, the pixel circuits 801 in whichthe data has been written are brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 854 is controlled in accordance with thepotential of the written data signal. The light-emitting element 872emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage is displayed.

Alternatively, the pixel circuit can have a function of compensatingvariation in threshold voltages or the like of a transistor. FIGS. 15Aand 15B and FIGS. 16A and 16B illustrate examples of the pixel circuit.

The pixel circuit illustrated in FIG. 15A includes six transistors(transistors 303_1 to 303_6), a capacitor 304, and a light-emittingelement 305. The pixel circuit illustrated in FIG. 15A is electricallyconnected to wirings 301_1 to 301_5 and wirings 302_1 and 302_2. Notethat as the transistors 303_1 to 303_6, for example, p-channeltransistors can be used.

The pixel circuit shown in FIG. 15B has a configuration in which atransistor 303_7 is added to the pixel circuit shown in FIG. 15A. Thepixel circuit illustrated in FIG. 15B is electrically connected towirings 301_6 and 301_7. The wirings 3015 and 301_6 may be electricallyconnected to each other. Note that as the transistor 303_7, for example,a p-channel transistor can be used.

The pixel circuit illustrated in FIG. 16A includes six transistors(transistors 308_1 to 308_6), the capacitor 304, and the light-emittingelement 305. The pixel circuit illustrated in FIG. 16A is electricallyconnected to wirings 306_1 to 306_3 and wirings 307_1 to 307_3. Thewirings 306_1 and 306_3 may be electrically connected to each other.Note that as the transistors 308_1 to 308_6, for example, p-channeltransistors can be used.

The pixel circuit illustrated in FIG. 16B includes two transistors(transistors 309_1 and 309_2), two capacitors (capacitors 304_1 and304_2), and the light-emitting element 305. The pixel circuitillustrated in FIG. 16B is electrically connected to wirings 311_1 to311_3 and wirings 312_1 and 312_2. With the configuration of the pixelcircuit illustrated in FIG. 16B, for example, the light-emitting element305 can be driven by constant voltage constant current (CVCC). Note thatas the transistors 309_1 and 309_2, for example, p-channel transistorscan be used.

A light-emitting element of one embodiment of the present invention canbe used for an active matrix method in which an active element isincluded in a pixel of a display device or a passive matrix method inwhich an active element is not included in a pixel of a display device.

In the active matrix method, as an active element (a non-linearelement), not only a transistor but also a variety of active elements(non-linear elements) can be used. For example, a metal insulator metal(MIM), a thin film diode (TFD), or the like can also be used. Sincethese elements can be formed with a smaller number of manufacturingsteps, manufacturing cost can be reduced or yield can be improved.Alternatively, since the size of these elements is small, the apertureratio can be improved, so that power consumption can be reduced orhigher luminance can be achieved.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, the number of manufacturing steps is small, so that manufacturingcost can be reduced or yield can be improved. Alternatively, since anactive element (a non-linear element) is not used, the aperture ratiocan be improved, so that power consumption can be reduced or higherluminance can be achieved, for example.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

EMBODIMENT 5

In this embodiment, a display panel including a light-emitting device ofone embodiment of the present invention and an electronic device inwhich the display panel is provided with an input device will bedescribed with reference to FIGS. 17A and 17B, FIGS. 18A to 18C, FIGS.19A and 19B, FIGS. 20A and 20B, and FIG. 21.

<5-1. Description 1 of Touch Panel>

In this embodiment, a touch panel 2000 including a display panel and aninput device will be described as an example of an electronic device. Inaddition, an example in which a touch sensor is used as an input devicewill be described. Note that a light-emitting device of one embodimentof the present invention can be used for a pixel of the display panel.

FIGS. 17A and 17B are perspective views of the touch panel 2000. Notethat FIGS. 17A and 17B illustrate only main components of the touchpanel 2000 for simplicity.

The touch panel 2000 includes a display panel 2501 and a touch sensor2595 (see FIG. 17B). The touch panel 2000 also includes a substrate2510, a substrate 2570, and a substrate 2590. The substrate 2510, thesubstrate 2570, and the substrate 2590 each have flexibility. Note thatone or all of the substrates 2510, 2570, and 2590 may be inflexible.

The display panel 2501 includes a plurality of pixels over the substrate2510 and a plurality of wirings 2511 through which signals are suppliedto the pixels. The plurality of wirings 2511 are led to a peripheralportion of the substrate 2510, and part of the plurality of wirings 2511form a terminal 2519. The terminal 2519 is electrically connected to anFPC 2509(1).

The substrate 2590 includes the touch sensor 2595 and a plurality ofwirings 2598 electrically connected to the touch sensor 2595. Theplurality of wirings 2598 are led to a peripheral portion of thesubstrate 2590, and part of the plurality of wirings 2598 form aterminal. The terminal is electrically connected to an FPC 2509(2). Notethat in FIG. 17B, electrodes, wirings, and the like of the touch sensor2595 provided on the back side of the substrate 2590 (the side facingthe substrate 2510) are indicated by solid lines for clarity.

As the touch sensor 2595, a capacitive touch sensor can be used, forexample. Examples of the capacitive touch sensor include a surfacecapacitive touch sensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor include aself-capacitive touch sensor and a mutual capacitive touch sensor, whichdiffer mainly in the driving method. The use of a mutual capacitivetouch sensor is preferable because multiple points can be sensedsimultaneously.

Note that the touch sensor 2595 illustrated in FIG. 17B is an example ofusing a projected capacitive touch sensor.

Note that a variety of sensors that can sense proximity or touch of asensing target such as a finger can be used as the touch sensor 2595.

The projected capacitive touch sensor 2595 includes electrodes 2591 andelectrodes 2592. The electrodes 2591 are electrically connected to anyof the plurality of wirings 2598, and the electrodes 2592 areelectrically connected to any of the other wirings 2598.

The electrodes 2592 each have a shape of a plurality of quadranglesarranged in one direction with one corner of a quadrangle connected toone corner of another quadrangle as illustrated in FIGS. 17A and 17B.

The electrodes 2591 each have a quadrangular shape and are arranged in adirection intersecting with the direction in which the electrodes 2592extend.

A wiring 2594 electrically connects two electrodes 2591 between whichthe electrode 2592 is positioned. The intersecting area of the electrode2592 and the wiring 2594 is preferably as small as possible. Such astructure allows a reduction in the area of a region where theelectrodes are not provided, reducing variation in transmittance. As aresult, variation in luminance of light passing through the touch sensor2595 can be reduced.

Note that the shapes of the electrodes 2591 and the electrodes 2592 arenot limited thereto and can be any of a variety of shapes. For example,a structure may be employed in which the plurality of electrodes 2591are arranged so that gaps between the electrodes 2591 are reduced asmuch as possible, and the electrodes 2592 are spaced apart from theelectrodes 2591 with an insulating layer interposed therebetween to haveregions not overlapping with the electrodes 2591. In this case, it ispreferable to provide, between two adjacent electrodes 2592, a dummyelectrode electrically insulated from these electrodes because the areaof regions having different transmittances can be reduced.

Note that for example, a transparent conductive film including indiumoxide, tin oxide, zinc oxide, or the like (e.g., a film of IT) can begiven as a material of conductive films used for the electrode 2591, theelectrode 2592, and the wiring 2598, i.e., wirings and electrodes in thetouch panel. Moreover, for example, a low-resistance material ispreferably used as the material of the wiring and the electrode in thetouch panel. For example, silver, copper, aluminum, a carbon nanotube,graphene, or a metal halide (such as a silver halide) may be used.Alternatively, a metal nanowire including a plurality of conductors withan extremely small width (e.g., a diameter of several nanometers) may beused. Further alternatively, a metal mesh which is a net-like conductormay be used. Examples of such materials include an Ag nanowire, a Cunanowire, an Al nanowire, an Ag mesh, a Cu mesh, and an Al mesh. Forexample, in the case of using an Ag nanowire for the wiring and theelectrode in the touch panel, a visible light transmittance of 89% ormore and a sheet resistance of 40 Ω/cm² or more and 100 Ω/cm² or lesscan be achieved. A metal nanowire, a metal mesh, a carbon nanotube,graphene, and the like, which are examples of a material that can beused for the above-described wiring and electrode in the touch panel,have a high visible light transmittance; therefore, they may be used foran electrode of a display element (e.g., a pixel electrode or a commonelectrode).

<5-2. Display Panel>

Next, the display panel 2501 will be described in detail with referenceto FIG. 18A. FIG. 18A corresponds to a cross-sectional view taken alongdashed-dotted line X1-X2 in FIG. 17B.

The display panel 2501 includes a plurality of pixels arranged in amatrix. Each of the pixels includes a display element and a pixelcircuit for driving the display element.

For the substrate 2510 and the substrate 2570, for example, a flexiblematerial with a vapor permeability of lower than or equal to 10⁻⁵g/(m²·day), preferably lower than or equal to 10⁻⁶ g/(m²·day) can befavorably used. Alternatively, materials whose thermal expansioncoefficients are substantially equal to each other are preferably usedfor the substrate 2510 and the substrate 2570. For example, thecoefficients of linear expansion of the materials are preferably lowerthan or equal to 1×10⁻³/K, further preferably lower than or equal to5×10⁻⁵/K, and still further preferably lower than or equal to 1×10⁻⁵/K.

Note that the substrate 2510 is a stacked body including an insulatinglayer 2510 a for preventing impurity diffusion into the light-emittingelement, a flexible substrate 2510 b, and an adhesive layer 2510 c forattaching the insulating layer 2510 a and the flexible substrate 2510 bto each other. The substrate 2570 is a stacked body including aninsulating layer 2570 a for preventing impurity diffusion into thelight-emitting element, a flexible substrate 2570 b, and an adhesivelayer 2570 c for attaching the insulating layer 2570 a and the flexiblesubstrate 2570 b to each other.

For the adhesive layer 2510 c and the adhesive layer 2570 c, forexample, materials that include polyester, polyolefin, polyamide (e.g.,nylon, aramid), polyimide, polycarbonate, an acrylic resin,polyurethane, an epoxy resin, or a resin having a siloxane bond can beused.

A sealing layer 2560 is provided between the substrate 2510 and thesubstrate 2570. The sealing layer 2560 preferably has a refractive indexhigher than that of air. In the case where light is extracted to thesealing layer 2560 side as illustrated in FIG. 18A, the sealing layer2560 can also serve as an optical element.

A sealant may be formed in the peripheral portion of the sealing layer2560. With the use of the sealant, a light-emitting element 2550 can beprovided in a region surrounded by the substrate 2510, the substrate2570, the sealing layer 2560, and the sealant. Note that an inert gas(such as nitrogen or argon) may be used instead of the sealing layer2560. A drying agent may be provided in the inert gas so as to adsorbmoisture or the like. For example, an epoxy-based resin or a glass fritis preferably used as the sealant. As a material used for the sealant, amaterial which is impermeable to moisture or oxygen is preferably used.

The display panel 2501 includes a pixel 2502. The pixel 2502 includes alight-emitting module 2580.

The pixel 2502 includes the light-emitting element 2550 and a transistor2502 t that can supply electric power to the light-emitting element2550. Note that the transistor 2502 t functions as part of the pixelcircuit. The light-emitting module 2580 includes the light-emittingelement 2550 and a coloring layer 2567R.

The light-emitting element 2550 includes a lower electrode, an upperelectrode, and an EL layer between the lower electrode and the upperelectrode. As the light-emitting element 2550, the light-emittingelement 100 described in Embodiment 1 can be used, for example. Notethat although only one light-emitting element 2550 is illustrated inFIG. 18A, it is possible to employ the structure including two or morelight-emitting elements.

In the case where the sealing layer 2560 is provided on the lightextraction side, the seating layer 2560 is in contact with thelight-emitting element 2550 and the coloring layer 2567R.

The coloring layer 2567R is positioned in a region overlapping with thelight-emitting element 2550. Accordingly, part of light emitted from thelight-emitting element 2550 passes through the coloring layer 2567R andis emitted to the outside of the light-emitting module 2580 as indicatedby an arrow in FIG. 18A.

The display panel 2501 includes a light-blocking layer 2567BM on thelight extraction side. The light-blocking layer 2567BM is provided so asto surround the coloring layer 2567R.

The coloring layer 2567R is a coloring layer having a function oftransmitting light in a particular wavelength region. For example, acolor filter for transmitting light in a red wavelength range, a colorfilter for transmitting light in a green wavelength range, a colorfilter for transmitting light in a blue wavelength range, a color filterfor transmitting light in a yellow wavelength range, or the like can beused. Each color filter can be formed with any of a variety of materialsby a printing method, an inkjet method, an etching method using aphotolithography technique, or the like.

An insulating layer 2521 is provided in the display panel 2501. Theinsulating layer 2521 coven the transistor 2502 t. Note that theinsulating layer 2521 has a function of covering unevenness caused bythe pixel circuit to provide a flat surface. The insulating layer 2521may have a function of suppressing impurity diffusion. This can preventthe reliability of the transistor 2502 t or the like from being loweredby impurity diffusion.

The light-emitting element 2550 is formed over the insulating layer2521. A partition 2528 is provided so as to overlap with an end portionof the lower electrode of the light-emitting element 2550. Note that aspacer for controlling the distance between the substrate 2510 and thesubstrate 2570 may be formed over the partition 2528.

A scan line driver circuit 2503 g includes a transistor 2503 t and acapacitor 2503 c. Note that the driver circuit can be formed in the sameprocess and over the same substrate as those of the pixel circuits.

The wirings 2511 through which signals can be supplied are provided overthe substrate 2510. The terminal 2519 is provided over the wirings 2511.The FPC 2509(1) is electrically connected to the terminal 2519. The FPC2509(1) has a function of supplying a video signal, a clock signal, astart signal, a reset signal, or the like. Note that the FPC 2509(1) maybe provided with a PWB.

In the display panel 2501, transistors with any of a variety ofstructures can be used. FIG. 18A illustrates an example of usingbottom-gate transistors; however, the present invention is not limitedto this example, and top-gate transistors may be used in the displaypanel 2501 as illustrated in FIG. 18B.

In addition, there is no particular limitation on the polarity of thetransistor 2502 t and the transistor 2503 t. For these transistors,n-channel and p-channel transistors may be used, or either n-channeltransistors or p-channel transistors may be used, for example.Furthermore, there is no particular limitation on the crystallinity of asemiconductor film used for the transistors 2502 t and 2503 t. Forexample, an amorphous semiconductor film or a crystalline semiconductorfilm may be used. Examples of semiconductor materials include Group 13semiconductors (e.g., a semiconductor including gallium), Group 14semiconductors (e.g., a semiconductor including silicon), compoundsemiconductors (including oxide semiconductors), organic semiconductors,and the like. An oxide semiconductor that has an energy gap of 2 eV ormore, preferably 2.5 eV or more and further preferably 3 eV or more, ispreferably used for one of the transistors 2502 t and 2503 t or both, sothat the off-state current of the transistors can be reduced. Examplesof the oxide semiconductors include an In—Ga oxide, an In-M-Zn oxide (Mrepresents Al, Ga, Y, Zr, La, Ce, Sn, or Nd), and the like.

<5-3. Touch Sensor>

Next, the touch sensor 2595 will be described in detail with referenceto FIG. 18C. FIG. 18C corresponds to a cross-sectional view taken alongdashed-dotted line X3-X4 in FIG. 17B.

The touch sensor 2595 includes the electrodes 2591 and the electrodes2592 provided in a staggered arrangement on the substrate 2590, aninsulating layer 2593 covering the electrodes 2591 and the electrodes2592, and the wiring 2594 that electrically connects the adjacentelectrodes 2591 to each other.

The electrodes 2591 and the electrodes 2592 are formed using alight-transmitting conductive material. As a light-transmittingconductive material, a conductive oxide such as indium oxide, indium tinoxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium isadded can be used. Note that a film including graphene may be used aswell. The film including graphene can be formed, for example, byreducing a film containing graphene oxide. As a reducing method, amethod with application of heat or the like can be employed.

The electrodes 2591 and the electrodes 2592 may be formed by, forexample, depositing a light-transmitting conductive material on thesubstrate 2590 by a sputtering method and then removing an unnecessaryportion by any of various patterning techniques such asphotolithography.

Examples of a material for the insulating layer 2593 are a resin such asan acrylic resin or an epoxy resin, a resin having a siloxane bond, andan inorganic insulating material such as silicon oxide, siliconoxynitride, or aluminum oxide.

Openings reaching the electrodes 2591 are formed in the insulating layer2593, and the wiring 2594 electrically connects the adjacent electrodes2591. A light-transmitting conductive material can be favorably used asthe wiring 2594 because the aperture ratio of the touch panel can beincreased. Moreover, a material with conductivity higher than theconductivities of the electrodes 2591 and 2592 can be favorably used forthe wiring 2594 because electric resistance can be reduced.

One electrode 2592 extends in one direction, and the plurality ofelectrodes 2592 am provided in the form of stripes. The wiring 2594intersects with the electrode 2592.

Adjacent electrodes 2591 are provided with one electrode 2592 providedtherebetween. The wiring 2594 electrically connects the adjacentelectrodes 2591.

Note that the plurality of electrodes 2591 are not necessarily arrangedin the direction orthogonal to one electrode 2592 and may be arranged tointersect with one electrode 2592 at an angle of more than 0 degrees andless than 90 degrees.

The wiring 2598 is electrically connected to any of the electrodes 2591and 2592. Part of the wiring 2598 function as a terminal. For the wiring2598, a metal material such as aluminum, gold, platinum, silver, nickel,titanium, tungsten, chromium, molybdenum, iron, cobalt, copper, orpalladium or an alloy material containing any of these metal materialscan be used.

Note that an insulating layer that covers the insulating layer 2593 andthe wiring 2594 may be provided to protect the touch sensor 2595.

A connection layer 2599 electrically connects the wiring 2598 to the FPC2509(2).

As the connection layer 2599, any of various anisotropic conductivefilms (ACF), anisotropic conductive pastes (ACP), or the like can beused.

<5-4. Description 2 of Touch Panel>

Next, the touch panel 2000 will be described in detail with reference toFIG. 19A. FIG. 19A corresponds to a cross-sectional view taken alongdashed-dotted line X5-X6 in FIG. 17A.

In the touch panel 2000 illustrated in FIG. 19A, the display panel 2501described with reference to FIG. 18A and the touch sensor 2595 describedwith reference to FIG. 18C are attached to each other.

The touch panel 2000 illustrated in FIG. 19A includes an adhesive layer2597 and an anti-reflective layer 2567 p in addition to the componentsdescribed with reference to FIGS. 18A and 18C.

The adhesive layer 2597 is provided in contact with the wiring 2594.Note that the adhesive layer 2597 attaches the substrate 2590 to thesubstrate 2570 so that the touch sensor 2595 overlaps with the displaypanel 2501. The adhesive layer 2597 preferably has a light-transmittingproperty. A heat curable resin or an ultraviolet curable resin can beused for the adhesive layer 2597. For example, an acrylic resin, aurethane-based resin, an epoxy-based resin, or a siloxane-based resincan be used.

The anti-reflective layer 2567 p is positioned in a region overlappingwith pixels. As the anti-reflective layer 2567 p, a circularlypolarizing plate can be used, for example.

Next, a touch panel having a structure different from that illustratedin FIG. 19A will be described with reference to FIG. 19B.

FIG. 19B is a cross-sectional view of a touch panel 2001. The touchpanel 2001 illustrated in FIG. 19B differs from the touch panel 2000illustrated in FIG. 19A in the position of the touch sensor 2595relative to the display panel 2501. Different parts are described indetail below, and the above description of the touch panel 2000 isreferred to for the other similar parts.

The coloring layer 2567R is positioned in a region overlapping with thelight-emitting element 2550. The light-emitting element 2550 illustratedin FIG. 19B emits light to the side where the transistor 2502 t isprovided. Accordingly, part of light emitted from the light-emittingelement 2550 passes through the coloring layer 2567R and is emitted tothe outside of the light-emitting module 2580 as indicated by an arrowin FIG. 19B.

The touch sensor 2595 is provided on the substrate 2510 side of thedisplay panel 2501.

The adhesive layer 2597 is provided between the substrate 2510 and thesubstrate 2590 and attaches the touch sensor 2595 to the display panel2501.

As illustrated in FIG. 19A or 19B, light may be emitted from thelight-emitting element through either or both of the substrates 2510 and2570.

<5-5. Method for Driving Touch Panel>

Next, an example of a method for driving a touch panel will be describedwith reference to FIGS. 20A and 20B.

FIG. 20A is a block diagram illustrating the structure of a mutualcapacitive touch sensor. FIG. 20A illustrates a pulse voltage outputcircuit 2601 and a current sensing circuit 2602. Note that in FIG. 20A,six wirings X1 to X6 represent the electrodes 2621 to which a pulsevoltage is applied, and six wirings Y1 to Y6 represent the electrodes2622 that detect changes in current. FIG. 20A also illustratescapacitors 2603 that are each formed in a region where the electrodes2621 and 2622 overlap with each other. Note that functional replacementbetween the electrodes 2621 and 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentiallyapplying a pulse voltage to the wirings X1 to X6. By application of apulse voltage to the wirings X1 to X6, an electric field is generatedbetween the electrodes 2621 and 2622 of the capacitor 2603. When theelectric field between the electrodes is shielded, for example, a changeoccurs in the capacitor 2603 (mutual capacitance). The approach orcontact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 2602 is a circuit for detecting changes incurrent flowing through the wirings Y1 to Y6 that are caused by thechange in mutual capacitance in the capacitor 2603. No change in currentvalue is detected in the wirings Y1 to Y6 when there is no approach orcontact of a sensing target, whereas a decrease in current value isdetected when mutual capacitance is decreased owing to the approach orcontact of a sensing target. Note that an integrator circuit or the likeis used for sensing of current values.

FIG. 20B is a timing chart showing input and output waveforms in themutual capacitive touch sensor illustrated in FIG. 20A. In FIG. 20B,sensing of a sensing target is performed in all the rows and columns inone frame period. FIG. 20B shows a period when a sensing target is notsensed (not touched) and a period when a sensing target is sensed(touched). Sensed current values of the wirings Y1 to Y6 are shown asthe waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and thewaveforms of the wirings Y1 to Y6 change in accordance with the pulsevoltage. When there is no approach or contact of a sensing target, thewaveforms of the wirings Y1 to Y6 change in accordance with changes inthe voltages of the wirings X1 to X6. The current value is decreased atthe point of approach or contact of a sensing target and accordingly thewaveform of the voltage level changes.

By detecting a change in mutual capacitance in this manner, the approachor contact of a sensing target can be sensed.

<5-6. Sensor Circuit>

Although FIG. 20A illustrates a passive type touch sensor in which onlythe capacitor 2603 is provided at the intersection of wirings as a touchsensor, an active type touch sensor including a transistor and acapacitor may be used. FIG. 21 illustrates an example of a sensorcircuit included in an active type touch sensor.

The sensor circuit in FIG. 21 includes the capacitor 2603 andtransistors 2611, 2612, and 2613.

A signal G2 is input to a gate of the transistor 2613. A voltage VRES isapplied to one of a source and a drain of the transistor 2613, and oneelectrode of the capacitor 2603 and a gate of the transistor 2611 areelectrically connected to the other of the source and the drain of thetransistor 2613. One of a source and a drain of the transistor 2611 iselectrically connected to one of a source and a drain of the transistor2612, and a voltage VSS is applied to the other of the source and thedrain of the transistor 2611. A signal G1 is input to a gate of thetransistor 2612, and a wiring ML is electrically connected to the otherof the source and the drain of the transistor 2612. The voltage VSS isapplied to the other electrode of the capacitor 2603.

Next, the operation of the sensor circuit in FIG. 21 will be described.First, a potential for turning on the transistor 2613 is supplied as thesignal G2, and a potential with respect to the voltage VRES is thusapplied to a node n connected to the gate of the transistor 2611. Then,a potential for turning off the transistor 2613 is applied as the signalG2, whereby the potential of the node n is maintained.

Then, mutual capacitance of the capacitor 2603 changes owing to theapproach or contact of a sensing target such as a finger; accordingly,the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor 2612 issupplied as the signal G1. A current flowing through the transistor2611, that is, a current flowing through the wiring ML is changed inaccordance with the potential of the node n. By sensing this current,the approach or contact of a sensing target can be sensed.

In each of the transistors 2611, 2612, and 2613, an oxide semiconductorlayer is preferably used as a semiconductor layer in which a channelregion is formed. In particular, such a transistor is preferably used asthe transistor 2613 so that the potential of the node n can be held fora long time and the frequency of operation of resupplying VRES to thenode n (refresh operation) can be reduced.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

EMBODIMENT 6

In this embodiment, a display module and electronic devices including alight-emitting device of one embodiment of the present invention will bedescribed with reference to FIG. 22 and FIGS. 23A to 23G.

<6-1. Display Module>

In a display module 8000 in FIG. 22, a touch sensor 8004 connected to anFPC 8003, a display panel 8006 connected to an FPC 8005, a frame 8009, aprinted circuit board 8010, and a battery 8011 are provided between anupper cover 8001 and a lower cover 8002.

The light-emitting device of one embodiment of the present invention canbe used for the display panel 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchsensor 8004 and the display panel 8006.

The touch sensor 8004 can be a resistive touch panel or a capacitivetouch panel and may be formed to overlap with the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch sensor function. A photosensor may be provided in each pixel ofthe display panel 8006 so that an optical touch sensor is obtained.

The frame 8009 protects the display panel 8006 and also serves as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed circuit board 8010. The frame 8009 mayserve as a radiator plate.

The printed circuit board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 8011 provided separatelymay be used. The battery 8011 can be omitted in the case of using acommercial power source.

The display module 8000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<6-2. Electronic Device>

FIGS. 23A to 23G illustrate electronic devices. These electronic devicescan include a housing 9000, a display portion 9001, a speaker 9003,operation keys 9005, a connection terminal 9006, a sensor 9007, amicrophone 9008, and the like.

The electronic devices illustrated in FIGS. 23A to 23G can have avariety of functions, for example, a function of displaying a variety ofdata (a still image, a moving image, a text image, and the like) on thedisplay portion, a touch sensor function, a function of displaying acalendar, date, time, and the like, a function of controlling a processwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Note that functions that can be provided for theelectronic devices illustrated in FIGS. 23A to 23G are not limited tothose described above, and the electronic devices can have a variety offunctions. Although not illustrated in FIGS. 23A to 23G, the electronicdevices may include a plurality of display portions. The electronicdevices may have a camera or the like and a function of taking a stillimage, a function of taking a moving image, a function of storing thetaken image in a memory medium (an external memory medium or a memorymedium incorporated in the camera), a function of displaying the takenimage on the display portion, or the like.

The electronic devices illustrated in FIGS. 23A to 23G will be describedin detail below.

FIG. 23A is a perspective view of a portable information terminal 9100.The display portion 9001 of the portable information terminal 9100 isflexible. Therefore, the display portion 9001 can be incorporated alonga bent surface of a bent housing 9000. In addition, the display portion9001 includes a touch sensor, and operation can be performed by touchingthe screen with a finger, a stylus, or the like. For example, when anicon displayed on the display portion 9001 is touched, an applicationcan be started.

FIG. 23B is a perspective view of a portable information terminal 9101.The portable information terminal 9101 functions as, for example, one ormore of a telephone set, a notebook, and an information browsing system.Specifically, the portable information terminal can be used as asmartphone. Note that the speaker 9003, the connection terminal 9006,the sensor 9007, and the like, which are not illustrated in FIG. 23B,can be positioned in the portable information terminal 9101 as in theportable information terminal 9100 in FIG. 23A. The portable informationterminal 9101 can display characters and image information on itsplurality of surfaces. For example, three operation buttons 9050 (alsoreferred to as operation icons, or simply, icons) can be displayed onone surface of the display portion 9001. Furthermore, information 9051indicated by dashed rectangles can be displayed on another surface ofthe display portion 9001. Examples of the information 9051 includedisplay indicating reception of an incoming email, social networkingservice (SNS) message, call, and the like; the title and sender of anemail and SNS message; the date; the time; remaining battery; and thestrength of an antenna. Instead of the information 9051, the operationbuttons 9050 or the like may be displayed on the position where theinformation 9051 is displayed.

FIG. 23C is a perspective view of a portable information terminal 9102.The portable information terminal 9102 has a function of displayinginformation on three or more surfaces of the display portion 9001. Here,information 9052, information 9053, and information 9054 are displayedon different surfaces. For example, a user of the portable informationterminal 9102 can see the display (here, the information 9053) with theportable information terminal 9102 put in a breast pocket of his/herclothes. Specifically, a caller's phone number, name, or the like of anincoming call is displayed in a position that can be seen from above theportable information terminal 9102. Thus, the user can see the displaywithout taking out the portable information terminal 9102 from thepocket and decide whether to answer the call.

FIG. 23D is a perspective view of a watch-type portable informationterminal 9200. The portable information terminal 9200 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, viewing and editing texts, music reproduction, Internetcommunication, and computer games. The display surface of the displayportion 9001 is bent, and images can be displayed on the bent displaysurface. The portable information terminal 9200 can employ near fieldcommunication conformable to a communication standard. In that case, forexample, mutual communication between the portable information terminal9200 and a headset capable of wireless communication can be performed,and thus hands-free calling is possible. The portable informationterminal 9200 includes the connection terminal 9006, and data can bedirectly transmitted to and received from another information terminalvia a connector. Power charging through the connection terminal 9006 ispossible. Note that the charging operation may be performed by wirelesspower feeding without using the connection terminal 9006.

FIGS. 23B, 23F, and 23G are perspective views of a foldable portableinformation terminal 9201. FIG. 23E is a perspective view illustratingthe portable information terminal 9201 that is opened. FIG. 23F is aperspective view illustrating the portable information terminal 9201that is being opened or being folded. FIG. 23G is a perspective viewillustrating the portable information terminal 9201 that is folded. Theportable information terminal 9201 is highly portable when folded. Whenthe portable information terminal 9201 is opened, a seamless largedisplay region is highly browsable. The display portion 9001 of theportable information terminal 9201 is supported by three housings 9000joined together by hinges 9055. By folding the portable informationterminal 9201 at a connection portion between two housings 9000 with thehinges 9055, the portable information terminal 9201 can be reversiblychanged in shape from an opened state to a folded state. For example,the portable information terminal 9201 can be bent with a radius ofcurvature of greater than or equal to 1 mm and less than or equal to 150mm.

The electronic devices described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thelight-emitting device of one embodiment of the present invention canalso be used for an electronic device which does not have a displayportion. The structure in which the display portion of the electronicdevice described in this embodiment is flexible and display can beperformed on the bent display surface or the structure in which thedisplay portion of the electronic device is foldable is described as anexample; however, the structure is not limited thereto and a structurein which the display portion of the electronic device is not flexibleand display is performed on a plane portion may be employed.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

EMBODIMENT 7

In this embodiment, the light-emitting device of one embodiment of thepresent invention will be described with reference to FIGS. 24A to 24C.

<7. Light-Emitting Device>

FIG. 24A is a perspective view of a light-emitting device 3000 shown inthis embodiment, and FIG. 24B is a cross-sectional view taken alongdashed-dotted line E-F in FIG. 24A. Note that in FIG. 24A, somecomponents am illustrated by broken lines in order to avoid complexityof the drawing.

The light-emitting device 3000 illustrated in FIGS. 24A and 24B includesa substrate 3001, a light-emitting element 3005 over the substrate 3001,a first sealing region 3007 provided around the light-emitting element3005, and a second sealing region 3009 provided around the first sealingregion 3007.

Light is emitted from the light-emitting element 3005 through one orboth of the substrate 3001 and a substrate 3003. In FIGS. 24A and 24B, astructure in which light is emitted from the light-emitting element 3005to the lower side (the substrate 3001 side) is illustrated.

As illustrated in FIGS. 24A and 24B, the light-emitting device 3000 hasa double sealing structure in which the light-emitting element 3005 issurrounded by the first sealing region 3007 and the second sealingregion 3009. With the double sealing structure, entry of impurities(e.g., water, oxygen, and the like) from the outside into thelight-emitting element 3005 can be favorably suppressed. Note that it isnot necessary to provide both the first sealing region 3007 and thesecond sealing region 3009. For example, only the first sealing region3007 may be provided.

Note that in FIG. 24B, the first sealing region 3007 and the secondsealing region 3009 are each provided in contact with the substrate 3001and the substrate 3003. However, without limitation to such a structure,for example, one or both of the first sealing region 3007 and the secondsealing region 3009 may be provided in contact with an insulating filmor a conductive film provided on the substrate 3001. Alternatively, oneor both of the first sealing region 3007 and the second sealing region3009 may be provided in contact with an insulating film or a conductivefilm provided on the substrate 3003.

The substrate 3001 and the substrate 3003 can have structures similar tothose of the substrate 102 and the substrate 152 described in Embodiment1, respectively. The light-emitting element 3005 can have a structuresimilar to that of any of the first to third light-emitting elementsdescribed in the above embodiments.

For the first sealing region 3007, a material containing glass (e.g., aglass frit, a glass ribbon, and the like) can be used. For the secondsealing region 3009, a material containing a resin can be used. With theuse of the material containing glass for the first sealing region 3007,productivity and a sealing property can be improved. Moreover, with theuse of the material containing a resin for the second sealing region3009, impact resistance and heat resistance can be improved. However,the materials used for the first sealing region 3007 and the secondsealing region 3009 are not limited to such, and the first sealingregion 3007 may be formed using the material containing a resin and thesecond sealing region 3009 may be formed using the material containingglass.

The glass frit may contain, for example, magnesium oxide, calcium oxide,strontium oxide, barium oxide, cesium oxide, sodium oxide, potassiumoxide, boron oxide, vanadium oxide, zinc oxide, tellurium oxide,aluminum oxide, silicon dioxide, lead oxide, tin oxide, phosphorusoxide, ruthenium oxide, rhodium oxide, iron oxide, copper oxide,manganese dioxide, molybdenum oxide, niobium oxide, titanium oxide,tungsten oxide, bismuth oxide, zirconium oxide, lithium oxide, antimonyoxide, lead borate glass, tin phosphate glass, vanadate glass, orborosilicate glass. The glass frit preferably contains at least one kindof transition metal to absorb infrared light.

As the above glass frits, for example, a frit paste is applied to asubstrate and is subjected to heat treatment, laser light irradiation,or the like. The frit paste contains the glass frit and a resin (alsoreferred to as a binder) diluted by an organic solvent. Note that anabsorber which absorbs light having the wavelength of laser light may beadded to the glass frit. For example, an Nd:YAG laser or a semiconductorlaser is preferably used as the laser. The shape of laser light may becircular or quadrangular.

As the above material containing a resin, for example, materials thatinclude polyester, polyolefin, polyamide (e.g., nylon, aramid),polyimide, polycarbonate, polyurethane, an acrylic resin, an epoxyresin, or a resin having a siloxane bond can be used.

Note that in the case where the material containing glass is used forone or both of the first sealing region 3007 and the second sealingregion 3009, the material containing glass preferably has a thermalexpansion coefficient close to that of the substrate 3001. With theabove structure, generation of a crack in the material containing glassor the substrate 3001 due to thermal stress can be suppressed.

For example, the following advantageous effect can be obtained in thecase where the material containing glass is used for the first sealingregion 3007 and the material containing a resin is used for the secondsealing region 3009.

The second sealing region 3009 is provided closer to an outer portion ofthe light-emitting device 3000 than the first sealing region 3007 is. Inthe light-emitting device 3000, distortion due to external force or thelike increases toward the outer portion. Thus, the outer portion of thelight-emitting device 3000 where a larger amount of distortion isgenerated, that is, the second sealing region 3009 is sealed using thematerial containing a resin and the first sealing region 3007 providedon an inner side of the second region 3009 is sealed using the materialcontaining glass, whereby the light-emitting device 3000 is less likelyto be damaged even when distortion due to external force or the like isgenerated.

Furthermore, as illustrated in FIG. 24B, a first region 3011 correspondsto the region surrounded by the substrate 3001, the substrate 3003, thefirst sealing region 3007, and the second sealing region 3009. A secondregion 3013 corresponds to the region surrounded by the substrate 3001,the substrate 3003, the light-emitting element 3005, and the firstsealing region 3007.

The first region 3011 and the second region 3013 are preferably filledwith, for example, an inert gas such as a rare gas or a nitrogen gas.Note that for the first region 3011 and the second region 3013, areduced pressure state is preferred to an atmospheric pressure state.

FIG. 24C illustrates a modification example of the structure in FIG.24B. FIG. 24C is a cross-sectional view illustrating the modificationexample of the light-emitting device 3000.

FIG. 24C illustrates a structure in which a desiccant 3018 is providedin a recessed portion provided in part of the substrate 3003. The othercomponents are the same as those of the structure illustrated in FIG.24B.

As the desiccant 3018, a substance which adsorbs moisture and the likeby chemical adsorption or a substance which adsorbs moisture and thelike by physical adsorption can be used. Examples of the substance thatcan be used as the desiccant 3018 include alkali metal oxides, alkalineearth metal oxides (e.g., calcium oxide, barium oxide, and the like),sulfate, metal halides, perchlorate, zeolite, silica gel, and the like.

Next, modification examples of the light-emitting device 3000 which isillustrated in FIG. 24B are described with reference to FIGS. 25A to25D. Note that FIGS. 25A to 25D are cross-sectional views illustratingthe modification examples of the light-emitting device 3000 illustratedin FIG. 24B.

In the light-emitting device illustrated in FIG. 25A, the second sealingregion 3009 is not provided but only the first sealing region 3007 isprovided. Moreover, in the light-emitting device illustrated in FIG.25A, a region 3014 is provided instead of the second region 3013illustrated in FIG. 24B.

For the region 3014, for example, materials that include polyester,polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate,polyurethane, an acrylic resin, an epoxy resin, or a resin having asiloxane bond can be used.

When the above-described material is used for the region 3014, what iscalled a solid-sealing light-emitting device can be obtained.

In the light-emitting device illustrated in FIG. 25B, a substrate 3015is provided on the substrate 3001 side of the light-emitting deviceillustrated in FIG. 25A.

The substrate 3015 has unevenness as illustrated in FIG. 25B. With astructure in which the substrate 3015 having unevenness is provided onthe side through which light emitted from the light-emitting element3005 is extracted, the efficiency of extraction of light from thelight-emitting element 3005 can be improved. Note that instead of thestructure having unevenness and illustrated in FIG. 25B, a substratehaving a function as a diffusion plate may be provided.

In the light-emitting device illustrated in FIG. 25C, light is extractedthrough the substrate 3003 side, unlike in the light-emitting deviceillustrated in FIG. 25A, in which light is extracted through thesubstrate 3001 side.

The light-emitting device illustrated in FIG. 25C includes the substrate3015 on the substrate 3003 side. The other components are the same asthose of the light-emitting device illustrated in FIG. 25B.

In the light-emitting device illustrated in FIG. 25D, the substrate 3003and the substrate 3015 included in the light-emitting device illustratedin FIG. 25C are not provided but a substrate 3016 is provided.

The substrate 3016 includes first unevenness positioned closer to thelight-emitting element 3005 and second unevenness positioned fartherfrom the light-emitting element 3005. With the structure illustrated inFIG. 25D, the efficiency of extraction of light from the light-emittingelement 3005 can be further improved.

Thus, the use of the structure described in this embodiment can providea light-emitting device in which deterioration of a light-emittingelement due to impurities such as moisture and oxygen is suppressed.Alternatively, with the structure described in this embodiment, alight-emitting device having high light extraction efficiency can beobtained.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 8

In this embodiment, examples in which the light-emitting device of oneembodiment of the present invention is applied to various lightingdevices and electronic devices will be described with reference to FIGS.26A to 26C.

<8. Lighting Device and Electronic Device>

An electronic device or a lighting device that has a light-emittingregion with a curved surface can be obtained with the use of thelight-emitting device of one embodiment of the present invention whichis manufactured over a substrate having flexibility.

Furthermore, a light-emitting device to which one embodiment of thepresent invention is applied can also be applied to lighting for motorvehicles, examples of which are lighting for a dashboard, a windshield,a ceiling, and the like.

FIG. 26A is a perspective view illustrating one surface of amultifunction terminal 3500, and FIG. 26B is a perspective viewillustrating the other surface of the multifunction terminal 3500. In ahousing 3502 of the multifunction terminal 3500, a display portion 3504,a camera 3506, lighting 3508, and the like are incorporated. Thelight-emitting device of one embodiment of the present invention can beused for the lighting 3508.

The lighting 3508 that includes the light-emitting device of oneembodiment of the present invention functions as a planar light source.Thus, unlike a point light source typified by an LED, the lighting 3508can provide light emission with low directivity. When the lighting 3508and the camera 3506 are used in combination, for example, imaging can beperformed by the camera 3506 with the lighting 3508 lighting orflashing. Because the lighting 3508 functions as a planar light source,a photograph as if taken under natural light can be taken.

Note that the multifunction terminal 3500 illustrated in FIGS. 26A and26B can have a variety of functions as in the electronic devicesillustrated in FIGS. 23A to 23G.

The housing 3502 can include a speaker, a sensor (a sensor having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), amicrophone, and the like. When a detection device including a sensor fordetecting inclination, such as a gyroscope or an acceleration sensor, isprovided inside the multifunction terminal 3500, display on the screenof the display portion 3504 can be automatically switched by determiningthe orientation of the multifunction terminal 3500 (whether themultifunction terminal is placed horizontally or vertically for alandscape mode or a portrait mode).

The display portion 3504 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken when thedisplay portion 3504 is touched with the palm or the finger, wherebypersonal authentication can be performed. Furthermore, by providing abacklight or a sensing light source which emits near-infrared light inthe display portion 3504, an image of a finger vein, a palm vein, or thelike can be taken. Note that the light-emitting device of one embodimentof the present invention may be used for the display portion 3504.

FIG. 26C is a perspective view of a security light 3600. The securitylight 3600 includes lighting 3608 on the outside of a housing 3602, anda speaker 3610 and the like are incorporated in the housing 3602. Thelight-emitting device of one embodiment of the present invention can beused for the lighting 3608.

The security light 3600 emits light when the lighting 3608 is gripped orheld, for example. An electronic circuit that can control the manner oflight emission from the security light 3600 may be provided in thehousing 3602. The electronic circuit may be a circuit that enables lightemission once or intermittently plural times or may be a circuit thatcan adjust the amount of emitted light by controlling the current valuefor light emission. A circuit with which a loud audible alarm is outputfrom the speaker 3610 at the same time as light emission from thelighting 3608 may be incorporated.

The security light 3600 can emit light in various directions; therefore,it is possible to intimidate a thug or the like with light, or light andsound. Moreover, the security light 3600 may include a camera such as adigital still camera to have a photography function.

As described above, lighting devices and electronic devices can beobtained by application of the light-emitting device of one embodimentof the present invention. Note that the light-emitting device can beused for lighting devices and electronic devices in a variety of fieldswithout being limited to the lighting devices and the electronic devicesdescribed in this embodiment.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

This application is based on Japanese Patent Application serial no.2015-033719 fled with Japan Patent Office on Feb. 24, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting element comprising: a pair ofelectrodes; and a first light-emitting layer and a second light-emittinglayer provided between the pair of electrodes, wherein the firstlight-emitting layer comprises a fluorescent material and a first hostmaterial, wherein the second light-emitting layer comprises a materialexhibiting thermally activated delayed fluorescence, wherein the firsthost material has one of an anthracene skeleton and a tetraceneskeleton, wherein a S₁ level of the first host material is higher thanthe S₁ level of the fluorescent material, wherein that the T₁ level ofthe first host material is lower than the T₁ level of the fluorescentmaterial, wherein light emitted from the first light-emitting layercomprises light emitted from the fluorescent material, wherein lightemitted from the second light-emitting layer comprises light emittedfrom the material exhibiting thermally activated delayed fluorescence,wherein a difference in peak wavelength between a first emissionspectrum of light from the first light-emitting layer and a secondemission spectrum of light from the second light-emitting layer islarger than or equal 0 nm and smaller than or equal to 30 nm, andwherein a peak wavelength of the first emission spectrum is shorter thanor equal a peak wavelength of the second emission spectrum.
 2. Thelight-emitting element according to claim 1, wherein the secondlight-emitting layer further comprises a second host material comprisinga first organic compound and a second organic compound, and wherein acombination of the first organic compound and the second organiccompound can form an exciplex.
 3. The light-emitting element accordingto claim 1, wherein the first emission spectrum and the second emissionspectrum each have the peak wavelength in a blue wavelength range. 4.The light-emitting element according to claim 1, wherein a ratio ofexcitons generated in the first light-emitting layer to excitonsgenerated in the second light-emitting layer is in a range of 0.9:0.1 to0.5:0.5.
 5. A light-emitting device comprising: the light-emittingelement according to claim 1; and a color filter.
 6. A lighting devicecomprising: the light-emitting element according to claim 1; and ahousing.
 7. A light-emitting element comprising: a pair of electrodes;and a first EL layer and a second EL layer provided between the pair ofelectrodes, wherein the first EL layer comprises a first light-emittinglayer and a second light-emitting layer, wherein the firstlight-emitting layer comprises a fluorescent material and a first hostmaterial, wherein the second light-emitting layer comprises a firstmaterial, wherein the first material is a material exhibiting thermallyactivated delayed fluorescence, wherein the first host material has oneof an anthracene skeleton and a tetracene skeleton, wherein a S₁ levelof the first host material is higher than the S₁ level of thefluorescent material, wherein that the T₁ level of the first hostmaterial is lower than the T₁ level of the fluorescent material, whereinthe second EL layer comprises a third light-emitting layer, wherein thethird light-emitting layer comprises a second material, wherein thesecond material is one of a phosphorescent material and a materialexhibiting thermally activated delayed fluorescence, wherein lightemitted from the first light-emitting layer comprises light emitted fromthe fluorescent material, wherein light emitted from the secondlight-emitting layer comprises light emitted from the first material,wherein a difference in peak wavelength between a first emissionspectrum of light from the first light-emitting layer and a secondemission spectrum of light from the second light-emitting layer islarger than or equal 0 nm and smaller than or equal to 30 nm and,wherein a peak wavelength of the first emission spectrum is shorter thanor equal a peak wavelength of the second emission spectrum.
 8. Thelight-emitting element according to claim 7, wherein the secondlight-emitting layer further comprises a second host material comprisinga first organic compound and a second organic compound, and wherein acombination of the first organic compound and the second organiccompound can form an exciplex.
 9. The light-emitting element accordingto claim 7, wherein the first emission spectrum and the second emissionspectrum each have the peak wavelength in a blue wavelength range. 10.The light-emitting element according to claim 7, wherein a ratio ofexcitons generated in the first light-emitting layer to excitonsgenerated in the second light-emitting layer is in a range of 0.9:0.1 to0.5:0.5.
 11. A light-emitting device comprising: the light-emittingelement according to claim 7; and a color filter.
 12. A lighting devicecomprising: the light-emitting element according to claim 7; and ahousing.
 13. A light-emitting element comprising: a pair of electrodes;and a first EL layer and a second EL layer provided between the pair ofelectrodes, wherein the first EL layer comprises a first light-emittinglayer and a second light-emitting layer, wherein the firstlight-emitting layer comprises a fluorescent material and a first hostmaterial, wherein the second light-emitting layer comprises a firstmaterial, wherein the first material is a material exhibiting thermallyactivated delayed fluorescence, wherein the second EL layer comprises athird light-emitting layer and a fourth light-emitting layer, whereinthe third light-emitting layer comprises a second material, wherein thesecond material is one of a phosphorescent material and a materialexhibiting thermally activated delayed fluorescence, wherein the fourthlight-emitting layer comprises a third material, wherein the thirdmaterial is one of a phosphorescent material and a material exhibitingthermally activated delayed fluorescence, wherein the first hostmaterial has one of an anthracene skeleton and a tetracene skeleton,wherein a S₁ level of the first host material is higher than the S₁level of the fluorescent material, wherein that the T₁ level of thefirst host material is lower than the T₁ level of the fluorescentmaterial, wherein light emitted from the first light-emitting layercomprises light emitted from the fluorescent material, wherein lightemitted from the second light-emitting layer comprises light emittedfrom the first material, wherein a difference in peak wavelength betweena first emission spectrum of light from the first light-emitting layerand a second emission spectrum of light from the second light-emittinglayer is larger than or equal 0 nm and smaller than or equal to 30 nmand, wherein a peak wavelength of the first emission spectrum is shorterthan or equal a peak wavelength of the second emission spectrum.
 14. Thelight-emitting element according to claim 13, wherein the secondlight-emitting layer further comprises a second host material comprisinga first organic compound and a second organic compound, and wherein acombination of the first organic compound and the second organiccompound can form an exciplex.
 15. The light-emitting element accordingto claim 13, wherein the first emission spectrum and the second emissionspectrum each have the peak wavelength in a blue wavelength range.
 16. Alight-emitting device comprising: the light-emitting element accordingto claim 13; and a color filter.
 17. A lighting device comprising: thelight-emitting element according to claim 13; and a housing.
 18. Thelight-emitting element according to claim 13, wherein a ratio ofexcitons generated in the first light-emitting layer to excitonsgenerated in the second light-emitting layer is in a range of 0.9:0.1 to0.5:0.5.